My name is Rusty and I don't do 360s, but I'm going to offer some advice anyway since all of the other freestyle pros appear to be out skiing this week.
First you should always plan your jumps by breaking them down into the ATML model: Approach, Takeoff, Manuever and Landing. On approach you need to determine the right speed versus the type of kicker available so that you will have enough air time to complete your spin. You'll also want to have a little wind up or prespin occur on approach. On takeoff, having edged (instead of flat) skis will enhance your rotation. Start the spin with your head and upper body, then let your lower body follow. For the maneuver, you'll find that doing two 180s is the easiest. Turn your upper body to face backwards then let your lower body catch up. When the lower body catches up, turn your upper body again to face forwards and then let the lower body catch up before landing. You can adjust spin rate by arm position. Closer the body increases spin rate, farther from the body decreases the rate. Landing with more down speed than forward speed and with flatter edges reduces the penalty for not being aligned perfectly forward on landing.
One of the elements of slope style is "easy style it". This means to look for ways to start small and work your way up. For 360's you can start with flat spins on the snow and focus on upper body rotation. Another easy style approach is to do small 180 jumps starting forward to backward, then backward to forward. When you're finally ready to do your first 360 attempt, try the jump regular at first to get a sense of speed and timing and the arc of the jump. Try pretending the windup as you approach the jump, do the pop and count the air time (one thousand one, one thousand two, one thousand three). You'll want at least 3 seconds so that at second one you're upper body will be backwards, at second two you're upper body will be forward and at second three you're lower body will be in position. It's easier to stall upper body rotation in the air than it is to finish lower body rotation on the ground so try to have a little extra air time margin for error. It's also a little easier to take off at an angle instead of straight down the fall line and rotate in the direction that has you turning downhill when landing.
Have someone with you so that they can observe what you're doing. It's hard to tell what you are really doing when you attempt a spin. Wear a helmet and make extra sure that your approach and landing are clear.