I have been playing pickups for a while now and basicily this is what most people say

2 scouts
2 soldiers
1 demo
1 medic

these classes are preffered as they can get to the middle point quicker than the rest. The soldier by rocket jumping and the demo by sticky jumping which also charges the uber .One of the most important parts of the game is learning the jumps so you can get good postions in the middle before the other team arrive .

scouts can change but normally just for attacking the last point maybe spy or sniper to try and take there medic down so you have uber advantage .
scouts can also change when defending last to either pyro or heavy if things are getting desperate.

1 soldier can swap to heavy but it will leave you short for the middle fight which is the most important battle of the game and the uber will take longer to charge.

demos and medics never swap

everybody should be aware of this before the match starts and then this stops confusion in matchs

If anybody thinks differently please post

Yes that's the usual setup for competative tf2.

Heavy, sniper, engineer and sometimes spy or pyro are classes used sometimes depending on the situation. Don't switch to them if you aren't sure how/whhen to use them tho.

Also:

http://www.pwnage.tv/ Their on demand section has some awesome tf2 matches covered.

http://www.gotfrag.com/tf2/demos/ download demos of top teams and watch them in your tf2.

http://www.quadv.com covers some of the UK based TF2 and there are some nice casts for other games too.

My introduction to comp play was http://www.teamfortress.tv/ The strats have changed a bit since then. The remember the Americans almost always used to field 2 sollie, 1 sniper, 1 medic 2 scout and the Europeans had much love for the Heavy...

http://www.e-rev.tv/ is another which covers TF2 across the pond, as well as some other games.