First off, to any rookies who may end up reading his, welcome to FIRST.

It's going to be one of the best things you will experience in your time as a student. People on this forum are here to help, so don't be shy to ask for it. (Search for a previous thread on it first though.)
As for tips:
Keep it simple.
Many people have already stated it, and it can't be stated enough.
Other pieces of advice (for a rookie team):
1. Use the KOP. I've seen a fair share of team, even veteran ones, who try to build their own drive train and end up with one less competent than the KOP. The KOP may not necessarily be the best, but it's reliable and allows you to focus on your mechanisms .
2. Read the manual.

Take it seriously, the last thing you want is to find out you come in conflict with the rules at competition. Make sure you give yourself a practice inspection before bag.
3. Make components (relatively) easy to replace. Things break. Accidents happen. You'll save yourself a lot of headache if things like motors, motor controllers, and wires are easy to access. Access holes/points are key.
4. Design within your limits. Don't design a robot you can't build or afford. Know what your team can and can't pull off. It wouldn't be very wise for a rookie team to do a swerve drive, for instance... Also, do order parts you think you will need ahead of time. Things go out of stock pretty fast during build season.
I'll post more later if I think of some.