Last night at Power Ranger and I's lesson, another one of the riders at my stable hopped on Ranger to see if he could get him to canter completely around the (very large) arena. He was successful, and even tackled some of the jumps I wasn't able to get. He later explained that I ride too tensely, and am too heavy in the saddle for my horse, and said I need to have a 'light seat.' I know what a light seat is, and I have accomplished it before, but it's been difficult. Does anyone have any advice for making it easier? I can't seem to get it :/

Without seeing you ride, I don't think I could see exactly what you need to do in order to improve your seat. A good seat, however, is correct and balanced. Since horses respond to very slight shifts in weight, if your seat is incorrect, it tells the horse the wrong thing - think of nervous riders who lean forward when the horse takes off with them. That certainly doesn't tell the horse to slow down, it tells them quite the opposite! 

I second what flakemusic says!  I know my biggest problem is breathing when I ask for an upward transition with Jackson.  He's taken off on me so many times that I become tense when I ask for a canter.  Apparently you can see my legs shorten because I get so tense!  I'm going back to lunge lessons (I've been riding for 14 years... but this is something I need to get over!!) and work on breathing, sitting back, and relaxing during my transition.  This may be something that will help you as well.   Is Power Ranger (great name by the way) a quick or hot horse?  If you are tense on him, but not on another horse, you may want to spend some time riding a horse you are more relaxed on, and try to transition that feeling back to your own horse.  Above all, remember to breath!!!!!

 If you're tense, then you're not flexible at your joints - elbows, knees, ankles, lower back. Those parts of your body should feel like they're jello. Think about when you're riding in a car and go over a pothole. If you're clamped down in the seat or try to brace yourself, it's going to bang you pretty hard. But if you relax and kind of go with it, it's not so bad. When you brace on a horse, not only do you make yourself and the horse uncomfortable, but you're actually encouraging the horse to stop.

Centered riding has a ground exercise where a pair of students take turns "riding" each other, literally playing horsie. If you could get someone to do this with you, I bet you'd feel the difference in a second.

Have you ever done the exercise where you trot around and alternate between sitting trot, posting trot, and half seat? Maybe that would work for you. Personally, I think of light seat as a half seat that's just barely touching down on the horse's back. But springy knees and ankles are essential.