The key is production. If you are already producing revenues consistently or are a known performer then you will have no problem asking for and receiving an increased percentage of commission.
I do not work for the base percentage at any program, I dont think any of the more established affiliates do. The amount of increase is subject to your bargaining ability and perceived value to the program. 5 % is a given, 10% can be attained fairly easily also. Once you start seeking 15% or more then you really have to have a proven track record with that group and be an agressive bargainer.
Other concessions such as guaranteed monthly placement fees and/or additional CPA bonuses on top of commission are often available once you are a proven performer. These wont be offered generally, so you will have to ask for them. Let the affiliate manager know that this is being provided by other programs and is one of the factors you consider when deciding who gets prime placements and exposure.
Guys bear in mind that these sort of perks and raises are reserved for affiliates who EARN THE PROGRAMS MONEY. If you do not yet produce commissions of $1500 – $2000 per month with a program on a consistent basis its best not to hit them up for a raise.