I've got a couple of wheels with fairly sizeable dings on the rims, which I'm about to try to bend back into shape.

Was thinking of tightening an adjustable spanner over a tea towel and levering it over.

Any other techniques I should know about first though?

That sounds about right. Don't work the metal though, try and get it where you want it first time.

Don't know if you have access to a grinder, but i've made an adjustable specially for the job. Ive ground the inner jaw back by about 3-4mm and radiused the outer jaw tip, this allows it to sit further onto the rim, and prevents it digging in on the outer surface.
Tips- don't try and bend too much in one place or you could end up with the rim cracking, work along the dent from the middle out in small steps.

Thank you guys, a couple of good pointers there.

Don't think I'll be able to make my own tool for the job, but will certainly follow both your advice on the actual bending.

Fingers crossed.