Post your Infiltration stratagies and tips here (This first post will be updated with time with the incoming information so stay "tuned")

If you are on the Bravo (blue) team, never get to close to the Alpha base, it's annoying. Also, it's always good to have all areas covered while playing INF in the blue team, so team chat is essential.
The red team, should also make sure to strike in groups: blue team will do nothing but wait so it's easier if you go in 1 or 2 groups.

First and foremost remember that this is a team-oriented game mode. As such do your best to work in team, and encourage others to do the same. Now onto the actual tactics.

Alpha (usually attackers)
Your mission, should you accept it (and you have no choise), is to infiltrate the enemy compound and besteal a precious piece of rag on a stick, carry it all the way back through the trecherous terrain and crossfire of your opponents and deposit it on top of your own stick'n'rag (ala flag). This is your main task, meaning things like kills should not be your top priority when playing this game mode. Concentrate on getting in, and getting out, alive and with the objective in your pants.
Most of the time red team spawns on lower grounds and has to climb a hill/fortress/whatever to get into blue base. Things that will allow you to penetrate enemy defenses are directed automatic fire, m79 jumps and outright recklessness. Yes, rushing the enemy lines riddled with machineguns is stupid, but so is playing computer games.
Once you have scaled the first obstacle another challenge presents itself: you are alone, with team a few screens behind you, and enemies respawning up ahead. It might be a good idea to hold your ground here, unless you can manage to get in and out by the time chivalry arrives. Going in groups of 2-3 players is a good idea, but going in waves of 1-1-1-... is even better! If you were to go in groups, enemy could be able to deal with the threat and you are back at square one. But if you were to go in waves, yes enemies will be able to easily cut down a lone attacker, but you or your buddy behind will cut them down after because they were reloading, looking for health/nades, or just cheering their little victory. Lets carry on...
Grab the objective and make your way out; secondary escape routes are encouraged, so is notifying your team which pathways to cover once you go by. Aint it nice when you are followed by a couple enemy soldiers unloading clip after clip at you, and dash past your own troops, who puncture, cut down, decimate and then /piss on your persuers? Another important thing about this is clearing the way for the objective carrier. If you tell your team which way your are going, the enemy will probably do the same, and any blues ahead of you will (hopefully) turn around and intercept you.
A good habbit is to 'escort' the flag carrier right upto the cap place, staying fairly close, so you can grab the flag for whatever the reason (camper with a barret right in your base, a stray shot to the back of the head, etc).

Bravo Team (usually campers)
Grab a weapon of choise, run out of spawn and lay suppressive fire in the direction of incoming enemies. You will be called all sorts of naughty names for playing defensively, which is only acceptable in this game mode and Only for this team. Your task is to prevent enemy taking your flower, and getting it back in case you fail. It is usually not too difficult to keep reds at bay, because you are often given advantage of altitude, terrain/cover and relative stability of your position. Dont be tempted, however, to simply camp in a shrub or on a rock spraying autos at nothingless - thats criminal waste of ammunition. Wait out untill enemy comes within range and then blast them. Failing that, respawn, and from a secondarry defense line, repeat. Good idea is to tell your team how many enemies are coming through which pathway when you are in limbo (ala dead) so they can present counteractive measures.
If the flag is taken - give chase, unless its already too late and its better to just find good firing possition or resuply and wait for the next wave. Teamchat is, again, essential.

A small hint compared to Ivan0's,

If you are on alpha team, quickly choose a weapon and charge as fast as you can at the beginning of a round. Blue will not have their defenses up quite yet, unless it is a one-route map.

If you are on bravo team and choose to camp, please make sure you are well hidden! You are trying to be a Sniper, not a sharpshooter or a marksman. This term applies to everyone, even those with machine guns and what-not, because snipers are units used for recon operations and have to be stealthy and well hidden, whereas sharpshooters/marksmen are not camouflaged.

on big maps where you can fly over people like in inf_Abel or the beach one, you can m79 boost your way out of most peoples sight, making it easier to avoid enemy contact.

always report where enemies are, they will help your teammates defend the flag more efficiently. Or it will help red know where campers lie at.

A cheap strategy in maps like fortress is to camp near the white flag and kill the flagger when hes about to score. To avoid it, you should have teammates help you kill him.

In inf_fortress its a good idea to hav a barret aiming at the statguns so that they dont get to use them.

I personally think that if you are on Alpha, and you've got teh objective, you shouldn't care about any opposition. Don't shoot at anyone, except M72-ers. Rush straight to the White Objective.