The best clans all have tactics, they work as a team to achieve the results, the UKCCL clans in Div 1 & Div 2 use tactics a lot. Which is why they are there. Clans need to get together and practice the maps they are going to play. Say you have five players playing:
Player 1 : Very good with Rail
Player 2 : Very good with Rocket Launcher
Player 3 : Good all rounder
Player 4 : Has trouble with ping
Player 5 : LPB, and is good with all guns
Most of the DM maps have the Rail Gun and the Rocket Launcher. So Player 1 would patrol the Rail Gun area and give out Rail Guns to other clan members who come near him with no good weapons.
Player 2 would patrol the Rocket Launcher area giving out RL's to clan members who pass by.
Players 3 & 4 would back up 1&2 providing cover and protection.
Player 5 would loop around the map going through players 1 & 2 positions getting the guns from them and kills anyone he meets on his loop, he would also go through the Quad and Pent areas to gain there extra powers.
Remember that some clans will use the same tactics as this (these are standard "win at all costs" tactics). So you will have to adjust to another plan if needs be. Try and work out tactics in a IRC chat session a couple of days before the big game.
Try and know the team you are playing, for instance if you know that none of the opposing clan can use the Rail Gun then don't patrol that area, just put extra men on Rockets.
Quake 2 Clan play gets very tactical, but here is a simple phrase to remember it by.
"The clan that controls the weapons controls the game"