I was so distrustful of the idea of 'the spark' that I married a man I didn't fancy in the slightest, thinking that because we were good friends that the rest would come. It never did and we divorced, although that was mainly due to the fact that he turned out to be a child in a man's body and a complete tosser to boot.
I dated for several years after that and the only time I felt an immediate, overwhelming spark turned out to be the most dangerous relationship I've ever had. After I managed to dump him he stalked me, the police had a record of it and it transpired he had actually been in prison for a similar offence before.
However. I went clubbing one night and was very, very drunk and saw DH dancing in the middle of the dancefloor and my first thought was 'Oh. There you are.' It was weird. I think all the booze had bypassed my normal lack of judgement skills
.
There was no 'spark' , as such (beyond normal first meeting quivers) but I just felt like I'd known him forever. In fact, the taxi-driver who took me home asked me how long we'd been a couple and was mighty surprised to find out it was only a few hours.
The spark, for me, grows more and more each year. I guess, by the time we're pensioners, I'll be all over him like a rash! 
One thing I did do (and I know how 'woo' this is going to sound, but hey ho) was write a list of things I wanted from a relationship, things I could compromise on, and things I definitely wouldn't put up with. It seemed to help, somehow. And DH fits all the right boxes (apart from the persistent watching of sport on the telly, but I neglected to think of that for my list, so can't complain.)