grappling rules are simple to me, and defined quite well by the players handbook.
If I remember correctly, natural weapons give the ability to initate a grapple without an aoo, but dont quote me on that, since i dont have my books up infront of me right now. There is a feat that allows for a free grapple attempt when you land a strike with a light melee weapon called skorpions grasp, i believe its in complete warrior or sword and fist.
but as far as the rules go, basically you use your standard action to initiate a grapple check with the creature, normally an opposed strength check moddifed by your grapple mod (generally your strength mod unless you have feats or magic to boost it) and the creature gains an attack of opportunity (aoo) against you. if the attack succeeds it adds to the grapple DC or could stop you from grappling altogether. hold on im getting my books open... i have to check this stuff.(5 min pause)
Starting a Grapple
To start a grapple, you need to grab and hold your target. Starting a
grapple requires a successful melee attack roll. If you get multiple
attacks, you can attempt to start a grapple multiple times (at
successively lower base attack bonuses).
Step 1: Attack of Opportunity. You provoke an attack of opportunity
from the target you are trying to grapple. If the attack of
opportunity deals damage, the grapple attempt fails. (Certain monsters
do not provoke attacks of opportunity when they attempt to
grapple, nor do characters with the Improved Grapple feat.) If the
attack of opportunity misses or fails to deal damage, proceed to Step
2.
Step 2: Grab. You make a melee touch attack to grab the target. If
you fail to hit the target, the grapple attempt fails. If you succeed,
proceed to Step 3.
Step 3: Hold. Make an opposed grapple check as a free action. If
you succeed, you and your target are now grappling, and you deal
damage to the target as if with an unarmed strike.
If you lose, you fail to start the grapple. You automatically lose an
attempt to hold if the target is two or more size categories larger
than you are.
In case of a tie, the combatant with the higher grapple check
modifier wins. If this is a tie, roll again to break the tie.
Step 4: Maintain Grapple. To maintain the grapple for later
rounds, you must move into the target’s space. (This movement is
free and doesn’t count as part of your movement in the round.)
Moving, as normal, provokes attacks of opportunity from
threatening opponents, but not from your target.
If you can’t move into your target’s space, you can’t maintain the
grapple and must immediately let go of the target. To grapple again,
you must begin at Step 1.
Grappling Consequences
While you’re grappling, your ability to attack others and defend
yourself is limited.
No Threatened Squares: You don’t threaten any squares while
grappling.
No Dexterity Bonus: You lose your Dexterity bonus to AC (if
you have one) against opponents you aren’t grappling. (You can still
use it against opponents you are grappling.)
No Movement: You can’t move normally while grappling. You
may, however, make an opposed grapple check (see below) to move
while grappling.
If You’re Grappling
When you are grappling (regardless of who started the grapple), you
can perform any of the following actions. Some of these actions take
the place of an attack (rather than being a standard action or a move
action). If your base attack bonus allows you multiple attacks, you
can attempt one of these actions in place of each of your attacks, but
at successively lower base attack bonuses.
Activate a Magic Item: You can activate a magic item, as long as
the item doesn’t require a spell completion trigger. You don’t need
to make a grapple check to activate the item.
Attack Your Opponent: You can make an attack with an
unarmed strike, natural weapon, or light weapon against another
character you are grappling. You take a –4 penalty on such attacks.
You can’t attack with two weapons while grappling, even if both are
light weapons.
Cast a Spell: You can attempt to cast a spell while grappling or
even while pinned (see below), provided its casting time is no more
than 1 standard action, it has no somatic component, and you have
in hand any material components or focuses you might need. Any
spell that requires precise and careful action, such as drawing a
circle with powdered silver for protection from evil, is impossible to
cast while grappling or being pinned. If the spell is one that you can
cast while grappling, you must make a Concentration check (DC 20
+ spell level) or lose the spell. You don’t have to make a successful
grapple check to cast the spell.
Damage Your Opponent: While grappling, you can deal damage
to your opponent equivalent to an unarmed strike. Make an opposed
grapple check in place of an attack. If you win, you deal nonlethal
damage as normal for your unarmed strike (1d3 points for Medium
attackers or 1d2 points for Small attackers, plus Strength modifiers).
If you want to deal lethal damage, you take a –4 penalty on your
grapple check.
Exception: Monks deal more damage on an unarmed strike than
other characters, and the damage is lethal. However, they can
choose to deal their damage as nonlethal damage when grappling
without taking the usual –4 penalty for changing lethal damage to
nonlethal damage (see Dealing Nonlethal Damage, page 146).
Draw a Light Weapon: You can draw a light weapon as a move
action with a successful grapple check.
Escape from Grapple: You can escape a grapple by winning an
opposed grapple check in place of making an attack. You can make
an Escape Artist check in place of your grapple check if you so
desire, but this requires a standard action. If more than one opponent
is grappling you, your grapple check result has to beat all their
individual check results to escape. (Opponents don’t have to try to
hold you if they don’t want to.) If you escape, you finish the action
by moving into any space adjacent to your opponent(s).
Move: You can move half your speed (bringing all others engaged
in the grapple with you) by winning an opposed grapple check. This
requires a standard action, and you must beat all the other individual
check results to move the grapple.
Note: You get a +4 bonus on your grapple check to move a pinned
opponent, but only if no one else is involved in the grapple.
Retrieve a Spell Component: You can produce a spell component
from your pouch while grappling by using a full-round action.
Doing so does not require a successful grapple check.
Pin Your Opponent: You can hold your opponent immobile for
1 round by winning an opposed grapple check (made in place of an
attack). Once you have an opponent pinned, you have a few options
available to you (see below).
Break Another’s Pin: If you are grappling an opponent who has
another character pinned, you can make an opposed grapple check
in place of an attack. If you win, you break the hold that the
opponent has over the other character. The character is still
grappling, but is no longer pinned.
Use Opponent’s Weapon: If your opponent is holding a light
weapon, you can use it to attack him. Make an opposed grapple
check (in place of an attack). If you win, make an attack roll with the
weapon with a –4 penalty (doing this doesn’t require another
action). You don’t gain possession of the weapon by performing this
action.
this is the books exact words on grappling and what happens during it. It may be of some help to you, but you probably already know this. My opinion is basically once you get them in a grapple, if you can hold the grapple and you have a weapon you can attack with (unarmed, natural, or light), you do so with a -4 pen, but i believe there is a feat to reduce that pen or even remove it. my idea was to have a spike full plate fighter that way when he grapples the armor itself deals damage without even rolling attack, since ive got them held onto the spikes constantly, but im not sure that works by the DnD rules, thats something the DM would have to rule on.