Could anyone give me a little advice on how to make the double kick on FL sound convincing, and decent? I try and I very rarely nail it, and when I do it's by accident, what is the best way of going about it?
[Gibson Explorer]
[Ibanez RG1XXV]
[Line6 PodHD]
[ENGL Retro Tube 100]
[Dunlop CryBaby]
Vary the velocities slightly is one possibility. Also, try looser quantization to add a more human feel if you aren't doing already.

I find that the samples are always of great importance too, get a nice sounding kick which isn't too artificial and is nice and clean. A little thing I occasionally do as well is actually use a couple of kick samples and alternate between the two, but you have to make sure they don't sound too dissimilar.