The skills and what I found so far is:
- Stamina: just run, sprint, cycle, swim, whatever. Just move and it will grow(not that fast though)
- Shooting: personally I found shooting range to be a really fast way to improve your shooting skills
- Strength: fist fighting
- Stealth: performing stealth kills
- Flying: probably just flying, didn’t bother about it yet
- Driving: got the most problems here. Playing as a Trevor I can’t get past 1.8 or 1.9 bars(which is 36-38%). Drove for hours around and still can’t get it any higher
- Lung Capacity: staying underwater for as long as possible can very slowly but surely increase your lung capacity
Am I correct with the ones I know? And what would you recommend for the other ones?
According to the official GTA 5 Strategy Guide, here is a comprehensive list:
Stamina: this skill is increased by 1% for every 17 meters (18 yards) ran, every minute swam, every minute cycled. Participating in triathlons is a great way to increase this ability.
Shooting: this is increased quicker by landing hits on enemies, scoring headshots and successfully completing Shooting Range challenges. This skill is increased by 3% for every Gold challenge completed, 2% for every Silver and 1% for every Bronze.
Strength: this is increased 1% for every 20 punches. Playing sports (golf, tennis, darts) also increases strength.
Stealth: this skill is increased by 1% for every 45 meters (49 yards) walked in Stealth Mode and by 1.5% for every two stealth kills.
Flying: flying for 10 minutes increases this skill by 1%.
Completing the following training at the Flying School increases this skill by 3% for Gold, 2% for Silver and 1% for Bronze: Training take off, Runway landing, Inverted flight, Knife flight, Flat hatting, Loop the loop, Sky diving, Drop zone.
Completing the following training at the Flying School increases this skill by 5% for Gold, 3% for Silver and 2% for Bronze: Touch down, Helicopter course.
Completing the Helicopter speed run training increases the stats by 6%/4%/3%.
Driving: this skill is increased by 1% for every 10 seconds of wheelie action and an additional 1% for every second spent in the air in a vehicle or four-wheel landing. Performing Stunt Jumps with four-wheel landings gives the greatest boost to this skill.
Lung Capacity: this stats is increased by 1% for every minute spent underwater.