With certs being 500 XP a pop now and the 700 cert point weapons coming out, knowing how to best acquire certs is more important than ever. It's not always the most glorious or exciting stuff; you'll often find yourself being Future Janitor and doing things other people forget about, but it's all points in the end and if you lust for that scout rifle or a shotgun you'll need to hunker down and do the dirty work.
In most cases, the easiest and least work-intensive way to make certs is by repairing the turrets on large bases; there are almost two dozen turrets on an amp station, nearly that many on a biolab with an outer perimeter like Saurva and half a dozen at the top of a tech plant if you take a teleporter or lift. This isn't even counting the outer camps on most large bases. Each is worth approximately a third of a cert and takes about a minute to repair, not accounting for your repair tool overheating. While mind-numbing, this is incredibly safe.
The other major on-foot method of getting XP without getting into the business of killing is going medic and following infantry pushes. Focus especially on revives; a revive and full heal is worth almost as much as a kill via headshot (110XP). You'll also get 5XP if they get a kill while being healed or soon after being healed. While doing this press Q and scan the horizon; if an enemy dies after you've spotted them, you will gain 15 XP. This also helps your teammates keep track of targets in a hectic fight.
Another way is to be the spawn point on the frontlines by moving a Sunderer to hotspots and deploying. Every time a teammate uses your spawn point, you will gain 2 XP, with an additional 20 XP if they score a kill before dying. You will also gain points from repairing your Sunderer; being a spawn point, it will inevitably draw fire. This, however, has recently been nerfed; Sunderers can no longer deploy within 200 meters of each other and it is difficult for a single engineer to keep a vehicle up under fire.
Getting on a Flash or in a fighter and capping minor bases is another classic that's suffered some serious nerfs lately. You can slowly take a small one control node base for 275 XP, 25 for the control node and 250 for the capture itself. If you manage to solo a three node base that's 575 XP. This mostly works on Indar, as it has the most small-sized bases of any continent. Just look for any quiet area and pick up a flash or fighter.
These are the easiest ways to get XP without relying on your player skill right now; they mostly require a sense of where the fighting is and isn't. The rest of the article deals with farming methods that rely somewhat on player skill.
By far the most effective way to collect XP in my experience is vehicle driving, a simple infantry kill in a tank is worth 120XP and the explosive shells let you kill several people at the same time; you also get points for any kills or assists your secondary gunner gets. You can reliably attract a gunner at warpgate simply by upgrading your secondary from the perfectly awful Basilisk to even a Kobalt. If you have a friend you trust to gun for you I would strongly suggest HE (VPC for Vanu) shells just for sheer ability to slaughter infantry; a good secondary gunner can easily make up for the loss of damage against armour, especially the Magrider with the Saron HRB.
If you have one or two friends with microphones and are willing to invest in a real belly gun for it, the Liberator is probably the fastest way in the game to make large amounts of XP. This however requires mastery of the games flying physics, dodging the horde of fighters that clog the sky, and finding an outdoor battle where you can make a difference, Esamir tends to be best for this, though no continent is outstandingly bad for it.
In closing I'd like to say getting XP is often a matter of looking at the margins and sneaking in 5 or 10XP on little things, it adds up. Look for consoles that are either destroyed or shorting/on fire, throw ammo packs where you can, and when you see a guy missing even 10% of his health or some dude landing to fix his fighter, go over and help him out.