If you have something scheduled or some expected outside force that'll take you out of a raid, try to let the raid leaders know beforehand to help determine the ability to replace and continue on. Sometimes an event will depend upon a certain number of a certain number of classes, or a certain number of players. 

Sudden departures can really hamstring a raid.


Also, when a raid is scheduled (The times are given in guild motds often enough to where it's not a surprise) do your best to be near the raid destination and on time. This not only helps the raid begin quicker and more efficiently, but it also tends to come across badly if you're the type who logs on way the heck out in the middle of nowhere asking for a summon.  

6:30 raid time does not mean log on at 6:30 and ask to be summoned while you finish taking care of something. Ideally it means we can start by very near 6:30 so that we have a good chance of being done before people start having to go.