Do it on your own time. Using the office kit to run off copies of your business plan may seem harmless, but remember: that's someone else's time and money you're using.
Talk it over with your boss. Be honest - without giving anything away.
It may cause bad feeling at first, but explain your thinking and you won't need to start begging if you need to call in for some tips.
Do your homework. Talk to people, study the market, do your sums - check that your idea is going to work. There will be a lot of eyes watching to see whether you can pull it off.
Mind how you poach. Make sure any clients who come with you do so for sound business reasons. They know what your more established rival can do.
Think before you cherry-pick. Using your old team can bring proven performers into the fold, but be sure that they're suitable. Don't offer Sally that swanky exec role just because she's a mate.
Consult your partner. Swapping a cushy nine-to-five for your own enterprise brings clocking-off problems and a heap of risks. Do it only if your beloved is fully on board.