A good practice starts with an outline of what needs to be covered. A few days prior to the practice, there should be an outline posted in the forums of what items will be covered, and how long each one should take. This gives everyone a chance to read and think of some ideas to bring to practice. The practice server can be any decent host and play server, no need for a low ping times in most practice sessions. Have everyone on voice communictions prior to practice, and have everything set up for when people come into the server. it's no fun to wait for 15 minutes while maps are loaded and voice channels are prepared. Make sure to set some basic rules for practices, or it can quickly get out of hand. People should wear battle tags during practice, just to get used to match conditions. Battle tags are abbreviated tags from normal game tags. "My Name-ABC" blocks a large area whereas "M" is easier to look around. People should know that nobody fires their weapons unless instructed, and there are certain staging areas to go to.
Some things that are good to cover in a practice are:
Movement Drills - Have people practice moving in groups of 2-4 people, coordinating their looking left/right, covering fire, and leapfrog movements.
Map Drills - See how long it takes to move from one spot on the map to another, and the safest routes to move on.
A personal story: When some of my teammates were moving towards the targets, they turned down the closer of two alleys, and made the final approach in an open area. By telling them to move down the second alley, the open area of the approach was cut down by almost 50%. We covered it in practice, and people got more targets.
Firing Range - Have people scope thier weapons in against a small group of opponents. Once you learn to shoot from one spot well, it becomes easier in a game or match.
Glitch Drills - Matches almost always involve some level of glitching with most teams. Have someone set up in the glitches, and see what it takes to get them. In most every match, people will say, "I never knew you could do that" at the end of the match.
Practices are essential for matches and to create a more effective team. Make sure to keep practices controlled, or it becomes a waste of time with people just running around shooting randomly.