What he said ^ I've been writing raps since I was 8, so I've had a lot of time to practice lol but eventually it becomes second nature where your rhymes will be very intricate without you even trying. I like to vary my flow as it keeps things interesting. I've been influenced by a lot of great MC's and I've also created flows pretty much out of thin air. My favorite MC of all time would have to be Pharaoh Monch, and if you've ever listened to him, his flows are crazy!
If you practice freestyling it should help too. Don't focus so much on rhyming, but on flowing and delivery. I would even go as far as to say intentionally try not to rhyme, try to mess up and recover without missing a beat, you may stumble across a new way to deliver a rhyme that you like.
Rakim used to draw margins on his paper to indicate where he wanted to stress rhymes.
A good friend of mine told me that to develop his rhyme patterns he studied Jazz time signatures, and he's one of my favorite rappers(although that may not mean much to you lol sorry I don't have an example to post from him at the moment).
I hope my 1 am ramblings helped somewhat! Lol