What's the fastest/easiest way to level up your skills?
Solution 1:
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.
Solution 2:
Once you get the hang of tennis, play a few sets with Michael and Trevor. One set seems to raise strength by at least 20%, and it can be done on easy mode. Takes way less time to get strength to 100 this way as opposed to punching people (and running from cops).
Solution 3:
Special
Skilled by using up your Special gauge (press the left and right stick simultaneously)
Stamina
Skilled by using up your Stamina. Simply moving won't do, you need to move fast. It shouldn't be too difficult to skill, as long as you mash the A button (360) or the X button (PS3) while on foot, on a bicycle, or while swimming/diving.
Shooting
Skilled by shooting things, this skill will come along naturally.
Fastest way of improving this skill is by visiting the shooting range, just like Michael recommended to Franklin.
Strength
Skilled by punching people.
It's possible that visiting gyms can help improve this skill faster than punching everyone.
Stealth
Skilled by moving around stealthily. Whether people see you or not, does not seem to matter. Just press the left stick and move around. Apparently, silent takedowns also add to the stealth skill.
Flying
Skilled by flying around with planes, helicopters, or even the parachute.
Fastest and easiest way of improving this skill is by visiting the flight school at the Los Santos airport.
Driving
According to the ingame description, this is skilled by performing perfect landings (when your land vehicle is airborne) or by doing wheelies.
Based on that, it seems like the easiest and fastest way of improving this skill, is by driving over uneven terrain (offroad) instead of regular roads, whenever possible.
Lung Capacity
Skilled by diving (possibly without diving suit).
Solution 4:
Stealth : go to the pier with the farris wheel and sneak behind people, kill them with a knife. Cops have a hard time getting out to you and the stealth kills quickly raise your stat.