I used to do the ol' sit on the bench first, try to man-curl it up to my lap from there approach, but when the weights started getting way too heavy I started basically doing db deadlifts and then sitting down with them. If you get "set" like this, I can't imagine you hurting your back unless your pressing more than you can deadlift, in which case there is no good way for you to get into position without help. From there just lean back and as momentum tips you over use your legs to help bring the dB's over your shoulders while you keep your arms straight. Ideally you can begin in the locked-out position and not deep in the "hole"; this will protect your shoulders and your back. If these tips don't help then your guess is as good as mine.
Are you sure your not making it sore during the pressing movement? Perhaps your back arch is causing pain, or maybe its just a healthy sore from retracting your scapula really hard (which is a good thing anyway)?