Props to English riders

I must give props to all you English riders lol.. I am a western rider and I have recently started jumping lessons and I honestly don't no how you guys do it first of all I can't even get my foot in the stirrup because there really short lol but I was wondering what kind of exercises you can do that Would be benificial especially for your thighs and abs because I don't have alot of muscle.. Any tips to improve my balance and posture in the saddle?

Try Pilates. :)

Lots of two pointing, with and without stirrups. It really builds up leg muscles, and helps your body figure out how to balance in that position

^Def! Also trot sets in two point! Also, crunches and just doing barn chores have helped me tons!

Lots of stretching...you will build up plenty of muscle riding...but you need to stretch those thighs as much as possible...I do yoga on my Wii fit

Pilates helps your core a lot. You could also do forward and side lunges.
There's also this video that has lots of good exercises to do under saddle.

How To Improve Your Classical Seat In Horse Riding (Sports & Outdoors: Horse Riding)

Everything everyone has mentioned is right on: working in a 2 point, stirrupless, etc. i'd also like to add that i've noticed that western riders who switch to english often have a tendancy to start off wanting to sit in a chair seat. So make sure that your leg is always correct so you are using and building up the correct muscles. There should be a vertical line from your hip down to your heel with your stirrup on the balls of your feet (outside branch should be slightly angled out so it's perpendicular to the horse). Your heel should be stretched down with more weight on the inside (big toe side) if your foot so your ankle also flexes slightly out.

It is a little different than western huh...lol...works totally different muscles when you jump! I have found NO exercises that help the EXACT muscles you use when riding. Sorry!! =) You can certainly do all of the above mentioned exercises that will make you a stronger person and help with your endurance, but the best way to work the muscles that you need to use while riding is to ride! Good luck! I hope you have fun!