What exercises/ stretches or things can I do to increse my stamina? I've already started a routine every other morning where i ride up and down a hill 5-10 times and i try to ride the trails at least 3 times a week.
Riding lots is good, and making sure to push yourself will help you improve.
For cross training, think about what you use mostly for XC riding: Cardio and foot speed, right? So find something that gives you that on the days you decided to give your ass a break from the saddle. Running will help with your cardio, particularly if you incorporate intervals.
Core training, and I'm guilty of not doing enough, is also a good way to improve overall performance. Even though, as cyclists, we don't actively engage our core on the bike, a strong core gives you a better platform from which to develope power.
Where the real gains are made, is in what you do AFTER your rides. I have the same routine every time: Shake, Shower, Stretch.
I start with a protein shake to get the protein that my muscles are looking for to begin rebuilding. I like to make this one with just water (as opposed to my morning shake that I make with 1% milk and half a banana), and use a protein powder that's low in fat and carbs.
After slugging back my shake I just in the shower.
Then I do a series of stretches. This is the one area I have focused on more than last year, and honestly it's made all the difference, though dropping 5lbs from my XC bike likely didn't hurt lol For stretching I focus on whatever caused me to think about it on my ride. If my quads were tight, calves cramped up, back hurt, I spend a little more time on them, but I always make sure to spedn time on all areas.