I flip it between Mrs, miss and ms. I don't use a set one, loads of places where I was registered before marriage (dentist, bank, gp etc) have me down as Miss and I've never altered it. Some new places I registered shortly after marriage I ticked Mrs. But now a few years on when filling in new forms I'll tick whichever box tickles my fancy.
Although reading through all the previous replies maybe my life would be simpler if I did a PhD.