I’ve recently started a spot trials on my uni but I’m finding it difficult. At the moment I am currently able to jump up a curb or a step but I’m finding it hard to jump any higher. If anybody has any tips for hoping with you seat out of jumping higher in general I would be very grateful.
I think what helped me most was just watching the technique, and practise practise practise!
having a bar or a ledge you can change the height of will help you a load too.
Here are some high seat in sidehops
http://www.youtube.com/watch?v=YUf7gU7mNh0
and here are some really high seat out front hops
http://www.youtube.com/watch?v=5SoG6KZuj_s
Work on both static hops (hops from standing still) and hops with a prehop (the ones from further away)
I would say when you can hop about 50-60cm comfortably with the seat in, move onto seat out front stuff 
it’s all about practise! good luck 
Thankyou for the help Alister
Unicycle mag issue 11 (September - November 2009) has an excellent article by Kris Holm on hops and gaps - covering static hops, SIF, difficult surfaces and more.
you must tuck :D:D:D