Depends is this for a specific NPC you have in mind or are you asking for ideas on enemies that can escape the party if needed.
Without magic, make a barbarian/rogue with dodge, mobility and the dash feat (+5ft MV from S&S). Also give him good ranks in tumble and light - medium armor. He is going to have a movement of 45ft and a +5 dodge bonus to AC against the PC he is fleeing from (if he fails his tumble checks), and with a full move he won't (usually) provoke AoO and will move 90ft, meaning the PC with have to make a full move to keep up. Give him a spiked chain as a weapon and then he will be the one getting to make the AoO against the PCs that follow after him.
Or if he runs he will move 180ft only provoking AoO if he fails tumble checks and then still has the +5 dodge bonus to his AC.
But in general, your NPCs will be screwed, movement makes a huge impact on battles I have found, and unless the environment and terrain can provide an advantage to the canny, then the faster person will always win an escape race.
CullAfulMoshuN...