for trials the best thing to do is squats, about 80% of your jumping power comes from thighs, squats also make your balance better and it trains your lower back and a lot more muscles in your body. you can start with 3 sets 10 reps deadlifts are also good for trials because you use your lower back in most of trials moves (gaps, taps, pedal ups etc.) hamstrings are also important
im doing this at first you can help with your hands and later you can even add some weightsand do something for your calves like calf raises on one leg you also need to eat healthy food and A LOT, im eating 4-6 meals every day + some smaller meals and snacks. drink water instead of soda and dont drink much juices its better to eat fruits than drink their sugar 3x10 squats 2x6 deadlifts 3x10 glute ham raise 3x20 calf raise (per leg) (70-90% of max weight is the best to improve strenght) dont train too much 2x a week is enough with 2 days of rest between workouts with 2 weeks you will notice extra centimeters in your sidehops oh and change your workouts after about 3 weeks because when your body gets used to it you wont get any better, its the same with heavy bikes it works at first but later it will only have negative effect good luck! edit: i forgot to add that plyometrics are also useful, im doing some plyometric excersises before weight training