Having a shootout agreement in place I think is the best option: At any moment, a founder may wish to buy someone else's equity, or may wish to sell his own. At that moment, he proposes a transaction value. The counter-party has the choice to take either side of the transaction.

Example: Alice wants out. She proposes that Bob buys her shares for 50k. Bob may either accept the purchase, if he thinks the price is correct, or he must sell his own shares to Alice if he thinks the proposal is over-valued.

There are more details to this, like a horizon allowing for funds to be sought, the possibility of financing by the company or dealing with more than two partners, but this is the gist of it.