Soma steel core levers (link) are excellent.
Most tyres can be fitted without levers, and it's absolutely nothing to do with brute force; it's all about technique.
Get one bead into the rim (should be simple enough even with heavy DH tyres), then insert the (empty) tube, push the valve in and get the other bead into the rim at the valve hole. With the wheel on the ground in front of you, use your hands like crab claws to push the bead over the rim, one hand feeding around to the right and the other off to the left. You'll get to a point where you can no longer push the bead over the rim because the tension is too great on the exposed section of bead. This is the point where people usually break out the levers and try forcing the bead into place…
Instead, go back with your claw hands to the valve hole. As you've been feeding the bead in up the rim, the tension has been building on both the exposed section of bead and the portion which is already fitted. So, from the valve, use your thumbs to push the bead in towards the center of the rim… you'll feel it pop away. Continue doing this up to the point where you were stopped earlier and you'll find that the tension has decreased enough to allow you to feed more bead over the rim. Every time you're stopped, go back top the valve and release the tension. Many tyres can be fitted entirely by hand using this method, although some may require the last few inches of bead to be pressed over with a lever.
If you're ever having trouble getting a tyre off, make sure all the air is expelled and then use the nipping technique to push the two beads into the center valley of the rim. In most cases, you'll find that it will make your job considerably easier, to the point where a lever is only required to lift the first portion of bead out over the rim.
