Got to be in the right place at the right time and have friends/contacts who want to work with you.
Drop everything to get it quoted asap, go in as competitive as you feel you can and then figure out how to make it work.
Don't expect to make a lot of money out of it, but it is very steady and reliable.
Some on here will probably contradict the above but just my experience.
nope I won't contradict, that pretty much sums it up 
I agree with the above.
I used to have a fair amount of commercial, I now concentrate on resi.
The problem with commercial can be that you have a few very large jobs, that take up most of your time, and make most of your money. The problem with that is that when said company pulls the contract it can be a jolt. I had a job go from just over a £1000 a month to £300, and another one some guy undercut me by about 40% and won it, that was £800 a month gone.
I am pleased that I always kept a proportion of resi work on the go otherwise I would have had a big problem.
But the monthly BACS transfer/cheque is nice to receive
I think it's worthwhile just improving your resi work myself, but maybe I'm just getting old and careful :
I got asked to quote a load of argos stores last year, they wanted them done for about £8 each, I kid you not. Someone will do it, and if they have loads of other work around it, it'll work for them, but no thanks