Recently, I was working on fighter training rules, inspired by Akrasia's attempt at the same and also AustroDavicus' rules on alternate magic-users. I received some feedback, and the main gripe with the system is that you can mix and match too many different trainings. I had considered putting some kind of artificial limit on what trainings are available to a character, but I've had a change of heart and decided to go a more intangible route- putting responsibility in the individual DM's hands.
Okay, so check this: a fighter begins with one training at 1st level. This is his training. This is what kind of fighting man he is. At 4th level, he will automatically take the Advanced version of his training, unless he has found a way to train with a different type of fighting man. What exactly constitutes training is left up to the DM. If your rules involve training costs, there's a guideline to go by. The DM might simply say that the fighter must spend a certain amount of game time with fighting men of that training; perhaps he needs to spend some time sequestered away with the reclusive Gray Mountain Brotherhood to gain access to the Warrior Monk training. Maybe a cavalier/knight spends some time with a reclusive old sword master and can now choose the Weapon Mastery training. The fighter doesn't actually get the benefits of the training until he reaches the next experience level at which he qualifies to take new training.
As for the demihuman race-classes, I've made these decisions:
1. The DM is well within his rights to say, Soup Nazi style, "NoTrainingForYOU" because you are already a special snowflake, and because maybe the DM wants a humanocentric campaign. (That seems to be a popular mode here in this corner of Blogaria)
2. Demihumans can choose from a limited list. My suggestion for stock-standard Tolkien demihumans would be:
Elves- Weapon Master (sword or bow only), Marksman, and perhaps Slayer (drow or orcs)
Dwarves- Berserker, Soldier, Weapon Master (Axe or hammer only), Slayer (orc, goblin, or giant, Veteran
Halflings- Weapon Master (sling or dagger only), Marksman, Veteran (the legendary halfling resilience)
Demihumans are eligible for trainings at the same levels that fighters are. DM's option as to whether or not they can receive training for things not on their list.
3. Tell me to get stuffed and allow any demihuman to take any training. Jeez.
Problem solved. Well, at least for me...
Disclaimer: These rules are generally intended for B/X. They don't play well with Cyclopedia's Weapon Mastery Rules, AD&D's weapon specializations, etc.