It's really down to these things.
Imagine you knew when you started what you know now
You've no float restrictions so you can use higher odds for SNR's
You've no fear because you know it works
When a good offer (e.g. Will Hill BOGOF's) comes along you can do it 6 (soon to be 7) times
Slots offers have notoriously wild variance but with 3 active Ladbrokes accounts you can smooth out the variance to make an overall profit on the week
And Finally, and in no small part, LUCK
With a new sign up I can now do all the lucrative UK bookies again but this time taking 90%+ from the sign ups. I no longer take a loss from qualifiers on mainstream books. I can put 20-25 hours per week into this so for the next few (active) weeks my figures should be good until I've done all the mainstream books again, and then in January I'm signing up the brother in law
As for luck, you make your own to some degree. When I place qualifying bets I tend to underlay them, especially on horses. In effect I'm giving up a few pence guaranteed profit in the hope that I'll benefit and make a few pounds instead. If you back as many horses a week as I do you're gonna hit some winners e.g. I made £119 from the Totesport £100 basket, despite all the free bets being SNR, because I had 4 under layed winners.
Another thing to remember is that if the free bet is £25, £25 is only the minimum stake you have to place to earn it. If you find a decent football arb stick £100 (or more) on it, you'll still get your £25 free bet but you'll be a couple of quid up before you use it