Here are some useful BTB CTF tips.
There is a very real opportunity at the very beginning of the match. Immediately head to the nearest vehicle and make an attack run. Lots of times the enemy team scatters to go collect weapons and vehicles and leave the base unattended. I'd say about a third of the caps that I've made have been in the first minute.
Blood Gultch - This is all about numbers, have at least one person on constant attack with the warthog. Drive it to the back or front of the base while holding a shotgun, pop in and get the flag and hop back in the warthog. You'll die 9 times out of 10, but in a CTF game, you'll easily get 20 tries. Drive on whichever side is opposite of their teleport exit, you don't want to run into them as they teleport to meet you. If you have a large team, you can post a sniper in the cliffs above their teleport exit. A gunner is nice, but optional. Don't worry about leaving the hogs there, they respawn quickly and more than likely the enemy will bring them back. It's best to have a shotgunner stay in the base and the sniper able to see it as well.
Infinity - Don't bother with the warthogs unless you're completely out of vehicles. Banshees are the best. Fly low, I'm usually only a few feet off the ground, and always go through the tunnels; banshees are big targets. Fly straight to the flag and drop down onto it, then right back in. Point the nose straight up to get over the lip, then head back as low as possible to take cover behind rocks. A spotter should stay in the middle by the teleports to be able to call out enemy attacks. If there are no banshees, the ghost is the next best choice. For some reason lots of people don't know that pressing the jump button brings up it's nose, which is necessary to be able to get over the ground smoothly and fast. Drive straight up to the flag and then out the other side.
Gephyrophobia - Station one, maybe two snipers in the teleports to keep the bridge clear and to be able to call out attacks in advance. The attack vehicle is the banshee and the best route is to go underneath the side paths, plopping the banshee down by the teleports and going in with a shotgun and over shield. Grab the flag and go back to your banshee. They'll likely chase you, so fly down under the side path so they don't have a shot at you.
Sidewinder - This is probably the most "think on your feet" map. Post the sniper in the teleports, and have a shotgunner in the back of the base. Alternate how you attack. I usually make my first attack through the tunnels, get both powerups and use a grenade to deny the other team theirs (most players don't realize that they can grenade them back.). Drop down off the cliff on the other side because it's more direct and try to get in there. The other route that I've found to be effective, especially when they are playing hard defense is to take a warthog in. Drive it straight through the door (a passenger with a rocket launcher helps too) and drive straight for the flag, fishtailing as much as possible to kill almost all of them. The other attack path is through the teleports and often you'll want to use multiple attacks at the same time, this is one of the few maps where you can actually do that. Once you get the flag, you really have to think about what to do, you should have taken notes where enemies were during your approach and the sniper needs to be talking to you during your pull. If enemies are right out front, go up and out the teleports. Your sniper should be able to hold them off while you run it around the long way. If you can get to the outside, see if you have top control. If your teammates are in control of the top, take the nearest vehicle and drive it (stay off of the ice.) If going through the middle is too dangerous, B-line for the tunnel and go left. Throw it through the barrier with the two powerups and turn and fight the enemies while your teammates come to pick up the flag. They can't get it back from the other side because grenades do not affect the flag.
Ice Fields - The bridge is the most important spot. Post two snipers and a tank to defend it. Snipers call out enemy movement along the bottom and have tanks waiting to meet them. The runner should go straight up the tunnel, hop out and grab the over shield on the bridge (and maybe rockets too if it's there.). Drive your ghost straight to the ramp and turn it as you get out so it's facing the right way. Grab it and go back through the tunnel. I literally never take it along the bottom, it's sniper city.
Danger Canyon - Alternate between attacking through the middle and in vehicles. Ghosts are the best running vehicle because of how agile they are over the rough terrain. You really need to get good at mashing A. Always turn the ghost as you get out of it. Depending on they type of resistance you passed on the way in, you may chose to go through the tunnel or back the way you came. When going through the tunnel, always drop down through the middle and grab the rocket launcher (unless they have snipe control of the other side, in which case you'll never get out of there) then straight out the center and through the tunnels to the flag. This is another map where a partner is useful. Drop down into the middle with the flag and have a teammate come up your ramp to the sniper spawn. Pass it to the other player, or run it up the middle.
Timberland - Everything other than the ghost is useless. The best route is along the edges. You can pick up invisible and/or rockets if you want. Drive it straight into the base, it fits perfectly through the door. Hop out, grab it and go out the other side. You're in the base for less than three seconds, so anything other than a three-shot, you can withstand. Have your sniper make sure your path back is clear, if a tank is in the way, go around.
Death Island - My favorite map. It's all banshee combat and low, fast flying. No matter which base you are at, go right and stay as low as possible. On one of the bases there is a tunnel to the right, go through that. If that's the side you'r attacking, thats also your attack and escape route. (Do not fly up high, snipers and pistols will take you down fast and give away your position.). Turn it around and grab the flag, then back to the right as low as possible. The side with the tunnel is easier to escape because the tunnel is faster cover. If your banshee gets taken, you're left with the teleports. Head towards the rocket launcher to stay as low as possible then go to the side of your base. On the red base, go to the right side and find the little slippery spot just above the door. Drop down there and make sure you hit the slant on the top of the door. On the blue side, go to the left and find the lowest spot. Face away from the base and drop down, hitting the little piece of cliff that juts out and the purple box. It's possible to do both of these without taking damage, but the blue side is very difficult. If you die during the fall, have a teammate pick it up.