How am I able to get extra experience when training a certain skill? For example, are there any items to modify the amount of experience I get for a certain skill?
NOTE: Since 8 October, items such as pendants and recharge gems no longer need to be equipped to give bonus experience. If you have a pendant (they were removed 31 October from the Squeal of Fortune), right click it and select "gain-xp". You will get bonus experience in that skill.
There are many items which do this, most which are skill-specific:
Bonus experience items for all skills: