Eat a diet high in soluble fibre (the soluble part is important). I have to, otherwise I get constipated.
Or drink gnarly scrumpy! Perry is even better. I recommend Black Rat on draught or Blakeney Red in bottles (not sure of their availability outside the west of England though). For maximum effect, alternate with pints of Cheddar Valley cider. I was so glad to be nappied the night I found that one out!
Or try an osmotic laxative like Movicol/Cosmocol. They work by drawing additional water into your gut so you need to drink plenty of water as well.
I'd say go for a softener rather than a pusher outer, seems kinder to your body. However if you do want a pusher outer, nothing beats sodium picosulfate for me, but take it easy! My daughter weighs 4 stone and takes 10ml daily for constipation on top of Movicol. I occasionally take 5ml if im stuck and it scares the shit out of me!
I have also tried her on liquid paraffin, which doesn't taste as revolting as it sounds, that was also good but not as good as sodium picosulfate. Lactulose tastes nice but did nothing for either of us.