a) The most important thing, which can actually increase your credit limit, is your credit score. If you have maintained a good and steady credit score, your credit line will be automatically hiked. Should you keep a good credit score, your limit will be higher or at your satisfactory level at the time apply for a card. The interest rates will also be lower then. So, try to keep a respectable score. You need not worry for your credit line then.
c) Make good use of your credit card regularly. Do not keep your card to use occasionally or only for emergencies. There must be a good number of transactions on your card for your issuer to understand your credit priority and hence, to increase your limit.
d) Do not just make your minimum payments every month. Try to make the full payment of your outstanding balance instead. This positive gesture surely pleases the creditors and thus, helps you get a quick increase in your credit limit. This will also help you avoid any finance charges, which are normally very exorbitant.
e) Do not be late in making your payments each month. Remember late payments not only bar you from using your card until you make the required minimum payments, it can drastically affect your chances to get an increase in the credit limit. Moreover, being late for a few months can result in the revocation of your credit privileges. You can no longer use your card but you have to keep paying towards your balance with finance charges. This will also influence your credit score negatively.
f) Approach your credit issuers to consider for an increase in your limit. Show them reasons as to why you deserve a hike in that. Mail them or call them or email them, just let them know that you have maintained your card very nicely and wisely. Hence, your limit deserves to be increased.
In fine, I would say, use your card wisely. Use it when you are in need of it. Do not possess too many credit cards. Keep good records of your cards. Pay your bills on time and never cross your credit line. You will for sure merit a credit line increase. All the best.