So first off, players should only get experience for active mitigation; basically, anything requiring planning and skillful execution. Negotiation (including bribes) falls into this category, along with active efforts to distract or defang an enemy. Simply spying on monsters and deciding not to engage doesn't cut it; there needs to be an actual plan requiring some active execution on the part of the group. This is the heart and soul of old-school gaming, with its emphasis on personal decision-making and problem solving:
Killing a monster nets its full experience point value because it's the riskier option; but a clever party can earn a percentage of this for non-violent solutions as follows:
BRIBERY nets 25% of a monster's experience point value plus 1 experience point per 10 gold pieces (or equal value) parted with. Victory always has its price.
EVASION awards 50% of total experience when a monster is aware of the party and the characters assume great personal risk. Avoiding passive enemies nets none.
TRAPS are another matter. Some rulesets grant experience for disarming these; and the decision to award advancement lies outside the scope of this posting.
Using Holmes Basic (the only book handy at the moment), successfully bribing an ogre with 500 gp would net 31 or 25% of the giant's listed value plus 50 (500 gp/10) for a total of 81, assuming combat is otherwise inevitable. Causing the ogre to give chase only to lose it in a nearby maze would net 62 experience or 50% of the monster's value; again, assuming the ogre would catch someone if the attempt failed - and that this was always the plan.
The above isn't perfect; but it's passable. And it lays the foundation for even better systems for rewarding a clever party. Old-school is killing; but it can be much more.
Some DMs think avoiding a dangerous encounter is its own reward. They could be right, obviously; it really depends on the tone of their campaign. But if old-school gaming is really about good strategy and tactics, it might be appropriate to reward a group with something extra for their cleverness, especially when in service to an overarching plan. Nothing should beat gold and kills; but this might be a good way to encourage the sort of behavior we old-school enthusiasts admire. Anyway, we'd love to hear how you handle this stuff...