Alchemy
In order to really maximize your skill gain, you should take advantage of either a Skill Tutor Statuette or a Crystal Ball of Knowledge. It will help keep your training in the "Optimal" difficulty range.
0 - 25: Buy from an NPC Alchemist
25 - 35: Make Agility Potions
35 - 45: Make Strength Potions
45 - 55: Make Cure Potions
55 - 65: Make Greater Agility Potions
65 - 75: Make Greater Strength Potions
75 - 85: Make Greater Heal Potions
85 - 95: Make Greater Cure Potions
95 - 100: Make Deadly Poison Potions
0 - 25: Buy from an NPC Alchemist
25 - 35: Make Agility Potions
35 - 45: Make Strength Potions
45 - 55: Make Cure Potions
55 - 65: Make Greater Agility Potions
65 - 75: Make Greater Strength Potions
75 - 85: Make Greater Heal Potions
85 - 95: Make Greater Cure Potions
95 - 100: Make Deadly Poison Potions