Skill practicing:

There are many factors that go into skill learning rates. These are outlined in the helpfile "Skill Improvement".

As far as actually practicing skills it depends more on the particular skill then anything else as to how you will go about doing it. Besides invokers, I think most people put effort generally only into specific skills such as parry, dodge, hide, camoflauged, creep, etc, all dependent on class. Each of these skills can be practiced in many ways. The following is how I tend to do them when I feel the need.

Defensive skills like parry, dodge, and shield block:

These skill get checked when you are taking hits from mobs or players. Thus to improve these skills you need to be tanking something. To perfect them at a low level (level 15 of above) the best way, in my opinion, is to maximize the number of attacks per round that you are tanking. To do this effectively you need to fight low enough level mobs so that you can sustain many rounds without needing to flee to rest. This way you are getting a large number of defensive skill checks per round providing many chance for the skills to improve. Generally the mobs are below "A perfect match" to make this work effectively.

Hints: For parry use a weapon good at parrying such as swords or staves. For dodge if possible go hand to hand. For shield block go hand to hand.

Weapon skills:

To specifically practice weapon skills use practice weapons, not because of any bonus but because they are low average damage. Find the most difficult mob that you can tank for many rounds that you will not kill. Attack the mob and do not use any offensive skills. It is ok to feint or dirt kick to lower the damage you are taking just do not kick for example. Do not kill the mob, flee and rest and repeat. Since the number of attacks you get per round has a low maximum range you want to increase the difficulty of the mob to increase the rate you are learning.

Hint: I wouldn't spam practice weapon skills in general. If you are going to practice skills as a melee class focus on defensive skills, your weapons will generally improve enough along the way.

Non-combat skills/spells:

Some skills and spells you can just spam practice without being in combat. Such as create water, earthquake, hide, sneak, or creep. Just find a quiet out of the way place and spam them as needed.

As for your question about low int races. Everything works the same as for characters with high ints, only it takes longer obviously. Personally the tedium of specifically trying to perfect skills at a low level with low int races tends to keep me from trying to do it. But, I get bored easily.