There are two benefits for a player character in most rps: money and power. Money is pretty self explanatory. You use it to get stuff in the equipment manual or ale and whores or whatever.
Power can cover a variety of categories but the catch all I use is it makes the character harder to mess with. It could be an alliance with an organized crime lord, a new set of armor that makes you harder to hurt or experience that improves your abilities. Improving your abilities is the most sought for quantity in most games I've played in because quite frankly, I as the gm don't take them away from my characters (99% true.) Equipment can be dropped, money can be stolen but if you're an expert with an auto pistol then you are an expert. Though I suppose the pistol can be stolen, all you need is to get your hands on another one.
The beauty of a game like Risus or Classic Traveller, my current favorites, is the simplicity of their systems yet those very systems also have a certain granularity to them. CT uses a 2d6 throw for most actions and a +1 there is a big deal. Risus uses dice pools of 1-6 d6 usually. An extra die is a big deal there as well; +3.5 towards success plus a big difference in combat situations.
One way to modify Risus (which I've shied away from) is to increase cliches by +1 at a time (not +1d6.) You could have up to +2 and the next increase would bump the cliche up a whole die. Thus 1d6, 1d6 + 1, 1d6 +2, 2d6. You could even do this in particularly important combats (which you don't mind dragging out.) First hit reduces 3d6 to 2d6 +2. This could also soften the Risus Death spiral. I wouldn't recommend it for team combat.
Another way of refining experience could be giving Quest Dice that specifically refer to what you did to earn the experience. Thus Hypertrophied Man might get one or more Questing dice in smashing giant robots because of his numerous battles with Major Mayhem's bots. When he gets 5 Questing dice he can convert them to a +1d6 for Strength or whatever his cliche is called. Lucky shots are another way of working this, you earn a Lucky Shot each time you increase in experience (maybe one die per adventure or roll against your highest cliche normally) and convert every three dice into a +1d6 for a cliche. Or keep a ton of Lucky Shots on hand.
Classic Traveller already has an experience system but many people are not thrilled with it. You basically make a determination roll to stick to your training and get one skill level or +1 to stat per four year cost. At the very least I'd advocate letting characters train in two skills simultaneously as they can gain 2 skills per term during generation.
Two years between skill increases is a little dragging though. Some refs might want to placate their players in the meantime by handing out 0-level skills. A 0-level skill can be very handy in some skills with a high negative dm for unskilled use such as Vacc Suit or Zero-G. You might want to restrict this to skills that don't require advanced training. For example it is doubtful you could pick up Pilot-0 just by watching a pilot or looking at online videos. Getting such advanced skills might also be restricted based on your current skills. For example, a trained pilot might pick up Ship's Boat-0 if he spends a lot of time on a shuttle.
A character could pick up a 0 level skill in 2d6 months if the situation warrants without a determination roll.