You've got just a couple of options, generally.
One is war. War, war, war. Crush your neighbors. Hopefully you're not so far behind the AI that you can't overtake them. The AIs are terrible at war (as I often mention) so just put siege units in your border cities with front-line troops between them and the enemy. Declare war, let them throw themselves on your swords or what have you, and when they beg for mercy, take their stuff. Lather, rinse, repeat.
I tend to puppet rather than annex, as that reduces the happiness hit. You can also raze and build on their turf, although this works best for tiny cities that are well placed. You'll also have to work fast if the AI could potentially settle in the cleared area before you get the chance.
The other possibility is to go for a sneaky cultural win. Cultural policies get harder to get the bigger you are. However, this can be difficult to do, depending on how far behind you are and how small you are. It's also harder in Civ5 than it was in Civ4, since your culture doesn't steal hexes in Civ5. (Do note: If you're playing with BNW, you need tourism, which is a totally different subject...)
Most of the other options are stymied by the fact that either your size limits you, or the AI will start to work against you as you approach victory.