Again I would advise anyone getting involved to be wary and only get started when you fully and completely understand what is involved. If you haven't used a betting exchange before for example then do some research. Although it is quite easy when you know how it can also be quite daunting.
Also I take issue with the above poster's statement about it being easy to make a grand a month.
Maybe back in the day but the layers have wised up a lot.
Once you have exhausted the big layer (coral etc) sign up bonuses. Further offers are few and far between and are usually based on loyalty like for example with 365 who'll give out the odd free bet or enhanced offer here and there.
Another thing to bear in mind is that most of these companies have links with each other nowadays.
Did you know Paddy and Betfair are the same company?
They get wise to matched bettors very quickly or arb abusers and you will end up having your account gubbed (limited or suspended).
Not trying to piss on anyone's parade just urging caution. Do the easiest offers, make some profit, enjoy it and don't be too greedy would be my final advice here.