Remember, too, that Traveller characters don't start out with no experience. Most PCs will have some money in the bank, decent weapons, and the skills to do the things they need to. So there isn't as much need to advance.
The only time I've ever experimented with an advancement system for Traveller, it was specifically tied to the skill system. Basically, for every skill task they succeeded by a certain amount, the player could get a "tick" or "check" mark for that skill. Once a skill had 10 x the skill level of them, they could make an education/intelligence roll to possibly increase that skill. Fail, and they had to wait to get another tick mark to try again. Succeed, and the skill went up by one, and the ticks got reset back to 0 for that skill.
A highly educated/intelligent character could relatively easily bring a skill up from 0 to 1 to 2, after that it got quite a bit harder.