Learn the Levels
One of the most important secrets to getting more points is to know the levels well. Focus on one map at a time, and take a few turns to learn the layouts. Find out what weapons are available to buy, and where; where the Mystery Box is located; and which locations are more easily defended and which place you at risk. Look for "Choke points" -- areas the zombies must funnel through en masse to get you. These are excellent point-scoring locations, but only as long as your back is covered by a teammate.
Controlled Breaching
Although a breach -- when the zombies break through a barrier and start swarming into your stronghold -- is generally bad, it can also be an advantage. When zombies die inside the compound, they sometimes drop power-ups, including double damage. If you let the zombies inside, you might get this, effectively giving you double the points you would normally get in that round. Be careful that your whole team is not using this strategy, or your defenses may be compromised.
Shooting
In earlier rounds, aim for the body, get multiple pistol shots off, then use your knife. This maximizes your point-scoring. In later rounds this is risky, so go for the kill instead. If two players are guarding an entrance, alternate your fire and you'll both score more points because you won't waste bullets on zombies that are already dead. The exception to this rule is when a large horde of zombies is going for you -- in this case, aim for head level and keep shooting.
Teamwork
Players with the best weapons should cover the main entrances, while others should cover windows. If you are lagging, cover a window for a while until your connection speeds up, so that the entrances are covered by people with better connections. Finally, at higher levels take the whole team to a "Camping spot" -- an area with only one way in -- and concentrate your fire. Only venture out between levels to reload.