Get yourself some sort of stand to work on so the tire is up off the ground. A plastic pail works well. Make sure you use plenty of soap to allow the tire to slide on the rim and into place. 20/80 soap to water should be more than enough. Install half the tire at a time. You're using tire irons right?
If it ends up being too much consider taking the rim and tire to your local dealer with the stipulation that you get to watch them install it. Better yet if you have a good relationship with your dealer ask if you can hang around for a couple hours and watch the pros do it a couple times then try it yourself. You'll be amazed how quickly some one with some practice can change a tire!
Just wanted to put in my two cents....maybe it'll help.
First off....gotta have "real" tire irons. The BIGGER the better. I'm using Magnum 15" irons. They're CHEAP!! About $8.00 each. Got all the right curves in all the right places. Make the whole job WAY easier.
Now...like Dave said...make sure you lube the bead of the new tire. I personally set my wheel on the floor on a hunk of scrap carpet . I kneel with the rim in front of me. I put one side of the tire on the rim . I then stuff my DEFLATED tube in the tire and fasten the valve stem lock nut. Keep in mind, the rim lock and rubber rim band never came off the rim and are already in position when putting the new tire on.
Okay... you're almost home. Here's the trick. When you're trying to get the last side of the tire on , MAKE SURE that the side you already put on does NOT seat against the far side of the rim yet. It must stay in the deepest center part of the rim. You know...the smallest diameter part of the rim. Now I start to work my way around the rim with the irons. I'll use my knees to hold the rim from moving by kneeling on a portion of tire already with both sides on the rim. Make sure to have your bead lube on hand if the bead starts to get dry. If it starts to get hard to pry the tire on at any point...double check the tire position on the rim...remember, CENTER. You should be able to work your irons around the rim with very little effort. Even at the last couple of inches of tire, it shouldn't require you to fight it. If it does, then you're doing something wrong.
After the tire is completely on the rim, I inflate the tube to about 40-50 psi. Just do this to seat the tire and make sure the tube straightens out in the tire. Now drop back to your normal tire pressure and tighten your rim lock. Do not tighten the rim lock before you inflate the tire. You'll end up with the tire out of round on the rim.
Hope this is clear enough to understand. It's really the kinda thing that is passed along better visually.
Also a trick for getting tire off ....if you need it just ask.
Now I can see why they say a picture is worth a thousand words.
P.S. If you like , you can email me and I'll give you my phone number. I can probably explain it better over the phone.:D
Kdx440 is right, make sure the side you already have on is not seated. I like to use three tire irons (Like he said the bigger the better) and add air when you put the tube in to streach it out and seat it in the tire, after let all the air out (put talcum or baby powder on the tube so it is lubricated ) work slow and easy so you don't pinch the tube