Probably all of these have already been covered off, sorry if it's a bit garbled..
Firstly, realistically, you only get to sign up to a bookie once. You need to make the most of that opportunity. If you dont (here speaks experience) you'll rue it in the future if you continue. If you havent got the time to do things properly, maybe hold off til you have.
Before you sign up, always check around for the best offer for the bookie. Paddypower, for instance, run a £250 free bets offer. When you sign up for less, you're giving money away.
Watch out for the expiry dates on any bets.
Don't underestimate the amount of float you need if you want to extract as much money as possible from the sites.
Also, check quidco/topcashback to see what they're offering, though some places wont let you use these with free bets.
Joining Betfair with a referral code from jobbingmusician on moneysaving expert, and going through quidco with get you around £50 in cash and free bets, if you make the bets criteria they set. jobbingmusician only keeps £5 of their referral bonus, returning the rest to you.
And , obviously, if others use your referral code, you can make free bets if they meet the minimum betting criteria.