The way Skyscraper just described is pretty much how I've been running our game. As long as the PC isn't running away from combat completely, they can move at will. If they want to turn tail and run, and haven't done anything to prevent the their opponent from doing so
(i.e turned over a table to get in the way, dumped over a flaming brazier), the opponent gets a free shot. i want to promote that swashbucking feeling in combat. Jump on tables, swing fromt he chandelier, kick sand in their face. I encourage my players to think out of the box, try crazy moves, but they also need to feel they can flee if needed...they just need to be smart about it.
We have ruled that certain activities, (anything taking a more than normal bit of concentration and focus, such as casting a spell or using a ranged weapon) doesn't give a free shot, but any attackers get a +d to their attack die, since the defenders attention is being spent on something other than melee.
Sometimes to decide this it takes a little discusion between myself and the players. They describe what they want to do, I try to assess its feasibility, give some pros and cons, and let the playeer decide it they want to try it. So much more freedom than having fixed rules that say "if the PC does this, then this will happen". You just got to get in the right mindset