Ways to Qualify for Sneak Attack:
1) Ambush: If you ambush your enemy, you get a free Surprise Round against them. A Flat Footed enemy loses their Dex bonus until they act. Remember the the Surprise Round is only a Standard Action. So you'll need Greater Manyshot or Pounce (remember that you can still Charge if your actions are limited) to get a full attack.
2) Win Initiative: If you win Initiative, you enemy is still Flat Footed, and still denied their Dex bonus.
3) Flanking: Have someone summon a lot of weak creatures.
4) More Flanking: Invest in Handle Animal. Buy a lot of dogs. They're cheap and easy to train.
5) Still More Flanking: Invest in Tumble, get behind your enemies, have your party's meatshield fight in front of them.
6) Yet More Flanking: Adaptable Flanker feat (PHBII) allows you to flank from any square. Combine with a reach weapon, and now you can stand next to or even behind a friend and still flank an enemy.
7) Tome of Battle Flanking: Island of Blades, a Shadow Hand stance allows you to flank from any square as long as you and an ally are both adjacent to the enemy. You can get this from a one level dip into Swordsage, or by taking the Martial Study -> Martial Stance feats.
Dear Gods, How Much Flanking Do We Need?: Obtain Familiar + Improved Familiar, if you can cast arcane spells. Now you have a full time friend to Flank with, and he can Share Spells with you (like Alter Self and Greater Invisibility).
9) Armor Lock: 1st level spell from Complete Scoundrel that works on enemies wearing armor. Buy a wand.
10) Greater Invisibility: Once your party hits level 7ish, there's really no reason someone in your group shouldn't cast this on you at the start of every combat.
11) Ring of Blinking: If you're party members are jerks and refuse to cast Greater Invisibility on you, use this item instead. Pick up the Pierce Magical Concealment feat (Complete Arcane) to ignore your 20% miss chance.
12) Skill Tricks: Again, check out the Complete Scoundrel. Skill Tricks can be very useful.
13) Fear: If your enemy is Cowering, he loses his Dex bonus. There are a large variety of ways to get and use Fear effects, and a variety of ways to corner or immobilize him.
14) Stun: If your enemy is stunned, he loses his Dex bonus. Work with the Monk in your party, or ask the caster to use spells with this effect.
15) Blind: If your enemy is blind, he loses his Dex bonus. There are spells and alchemical items that do this.
16) Helpless: There are a variety of spells and a few effects that render your foe paralyzed or otherwise helpless. A Rogue's Coup de Grace almost never fails.
17) Hide in Plain Site: There are many ways to get this. My favorite is a dip into
Warlock let's you Hide in Plain Site every round as a Swift action. This means that any enemy who fails their Spot check is denied their Dex bonus against your next attack. Not efficient if you want to make full attacks, but helpful nonetheless.
18) Grappling: An opponent who is grappled loses their Dex bonus to everyone except the grappler, another way to tag team with your party members.
19) Net, Razor Net, Lasso: Each of these is a touch attack that imposes a -4 penalty on Dex. Penalties from different sources stack. Enemies with 0 Dex count as being paralyzed. I wouldn't even bother with taking the Exotic Weapon feats, because touch attacks are easy, so the -4 penalty to hit is palatable. Though I would definitely invest in Spell Storing weapons, and find spells that deal Dex damage/penalties.
20) Feint: This is a retarded waste of an action in most cases. But it works well at low levels if you're not using TWF, and if you're an Invisible Blade with the Surprising Riposte feat (Drow of the Underdark), it works for a full attack.
21) Telling Blow (PHBII): When you crit, you also deal Sneak Attack. I'm not a fan of this method. It doesn't double your Sneak Attack if you flank and crit, WotC has made it clear that it just let's you qualify. So at best 30% of your attacks get Sneak Attack. There are many better uses for your feats, IMO.
22) More Tome of Battle Craziness: There are a bunch of manuevers which render your enemy Flat Footed or otherwise deny them their Dex bonus, especially in the Tiger Claw and Shadow Hand disciplines. You can also get Sneak Attack via the Assassin's Stance, which still qualifies you for the best Sneak Attack feats (Staggering Strike, Craven, etc). So in many ways a Swordsage is a better Sneak Attacker then the Rogue. (Or you can go Rogue 1/Swordsage X or Swordsage X/Nightsong Enforcer 1 so that you can use other stances).