Traveller has its own (very slow) advancement system, and to a degree this is necessary.
As described above, you can't have a PC three months out of his third term gaining more skills than a 7 term veteran. I think whatever system you use has to fall in line with the chargen advancement.
I've used a simple 'anniversary present' system. The later CT books have chargen providing typically 1 skill per year, so after a game year, I allow each character to gain one skill of their choice from a list of skills they've frequently or memorably used.
If they've picked locks every week for the past year, that would be a candidate for improvement on the basis of frequency. If one time they used their Survival skills to walk out of a desert, that would be a candidate on the basis of memorability.
Even that breaks the CT Bk 4/5/6/7 rate, Icos.
Remember: there's a skill gain roll in those books each year. Typically, 7+ (tho it ranges from 5+ to 13+ {aka "none"}). Most PC's will get about 2.6 skills per term in advanced CGen, and about 1.6 in basic CT. Also, Bk4/5/6/7 doesn't grant skills on promotion, only on school assignments or on the skills throw.
Note also: This got me thinking about MT's advancement rates.
MegaTraveller's stock 180 days per AT, and need to have 2AT to have a chance to raise a skill, and a 1/36 chance per month with only 2AT and Int 5-9...
So, for a character's prime skills...
Y0.5 1AT, no chance of raise.
Y1.0 2AT,
y1.5 1-(35/36)^6 ... 15% chance of raise. Gain 3rd AT at end
y2.0 1-(33/36)^6 ... 40% chance of raise; cum 49% chance. gain 4th AT at end
Y2.5 1-(30/36)^6 ... 66% chance of raise; cum 83% chance of raise. Gain 5th AT.
Y3.0 1-(26/36)^6) ... 85% chance of raise; cum 97.6% chance of raise.
So, the very lucky might make it annually; the very smart make it even faster (about 6 months faster). Most will make a level in 2.5 years.
MT thus allows for 2-4 levels per term in a single skill... but fails to limit the number of skills collecting AT's at once.