1. Don't reinvent the droid.
If you think you have a new idea, or want to do something crazy, just don't. Nearly all (call it 99%) of people who show up to the club with some new or exciting (to them) idea fall away very quickly. I've seen all sorts of talk about 'my droid will do this' or 'my droid will do that.' Almost none of them are around today. This goes for custom droids too. Think you are special? I think you aren't.
2. Follow in the footsteps of successful builders.
Don't buy anything from a parts supplier who hasn't finished a droid you would be proud of. Stick to Senna wood frames, JAG/Com8 aluminum frames, 300mm or R&J domes, etc. These parts are all PROVEN. A wood frame and a styrene dome are good bets even if you are on a budget. The Dave E plans have been used to completion multiple times too. There are some parts out there that lots of people buy, but few have actually been used in completed droids; this is a sign. And this should go without saying: If something isn't accurate to club spec, don't buy it! 18.5 inch bodies are not club spec!
3. Only buy council approved parts or from reputable club sellers.
Don't buy on eBay, don't buy on the RPF, don't buy from guys who sell their stuff on their own website. Buy from people who post on the forums, for the club. Ask anyone who was around for the Daniel Deutsch drama... the council is around for a reason. He sold his parts on his website, without any council oversight. His radar eye turned out to be inaccurate and people got ripped off.
4. Find a local builder who has completed a droid already.
If someone nearby you has already built a droid, then they've also probably got a lot of advice... and made a lot of mistakes. Learn from them; leverage their experience. People who build their droid in a vacuum aren't nearly as successful as people who had a local expert.
5. Don't try 2-3-2.
Do you want to actually finish your droid some day soon? Then don't try 2-3-2. Most (call it 90%) of people who attempt 2-3-2 fail. If you really want to set yourself up for heartache and failure, go right ahead and try. If you want a dependable, more affordable, faster-to-completion droid, then make a 3 legged droid from the outset. Still wondering what 2-3-2 is? Its the automatic transition from 2 legs to 3 legs.
6. You don't know what you don't know.
I was building a droid for over a year before I worked with Daren to complete my droid. All that time, I thought I had it all figured out. I had worked it all out in my mind.
Then, in the last month, we made a big push to get the droid done. When putting the whole droid together and finally getting him moving, I realized that I didn't know anything. I had no idea what was still ahead of me. I assumed things would fall together as planned, only to realize they wouldn't. This taught me one thing: until you finish the droid, you just don't know it all. Thats why its important to talk to and follow in the footsteps of those who have done it before.