Getting more certs is basically down to getting more xp. Many of these most people will already know but listed for those who are new.
Make sure you are within the area that gives you capture xp for facilities when they cap(where you can see the cap progress bar) particularly the big facilities.
Join a squad. You get bonus xp for actions that effect a squadmate.
Deploy a sunderer with an ams in a good location near heavy fighting. This can rack up quite a bit over long fights if you keep it alive.
Have an ammo resupply sunderer accompany a tank group, you will gain a fair ammount of xp from resupplying.
As an Infiltrator hack terminals and base turrets. Note that there is a limit on how many hacks of a type will give xp in a short time period, IIRC around 6 terminals and 6 base turrets before you need to wait a while before getting xp again.
As an Engineer repair everything. Just repairing all the turrets and terminals on a tower will usually give you 2-3 certs. Hang back a little from a tank push and repair them as they fall back damaged. Accompany Max groups and keep them repaired.
Also as Engineer, remember to drop your ammo boxes for resupply xp. Drop them near Heavys who are spamming rockets, next to capture points, near to choke points, etc. Also remember you have two of them by default, pressing b when you pull out your turret gives you a ammo pack that will last for 3 mins, press b again to switch back to turret.
As a Medic revive anyone who is dead, particularly Maxs, and heal anyone who needs it. Just watch out not to run into a kill zone trying to revive someone.
If you are in a tank/are a Heavy destroy base turrets, you get the same xp as scoring a kill. Just make sure that a friendly Infiltrator isn't intending to hack them.
Spot enemies. If you are the first to spot them then as long as they are killed while your spot is active you gain some bonus xp.
Finally shoot enemies. As long as they die you will usually get at least some xp for kill assist even if you don't kill them yourself.
The other way of getting a small ammount of certs is by killing a certain number of enemies with a specific weapon. Check on your stats page for how many you need to get for a cert reward.