The more Sanders loses the less leverage he'll have. Right now he has something Joe Biden and the Democratic Party want, a swift end to the primary. Now he can continue to ask people for donations as his position (and hopefully reputation) erodes or he can try to extract something meaningful while accepting reality.
Domestic legislative outcomes are going to be a wash and Biden has already made overtures in this area; however, foreign policy is a place where the executive gets to act with far more leeway. So, Sanders should be pushing to make his list of SOS nominees the list Biden chooses from. Hell, have Biden nominate him to be SOS it's whatever (look at the shyt Hillary did here). At the very least he could block the truly awful choices.
Tragically, that type of maneuver seems beyond his ability...