If you have a good relationship with them they will understand,and if you don't they would probably leave for a lower price in the future when approached by a competititor of yours anyway. Most people understand the costs of doing business, since they are in business themselves and realize those costs go up,it is most probably a bigger deal to you in your head(as most problems are) than it will be to them.Just bring it up in conversation,and tell them your price for _________ is now $******,if they badger you why,tell them fuel,insurance,labor costs etc.The agony of worrying about telling them is worse than telling them,and if you lose them as an account,like you said,you have better paying work waiting in the wings.I just upped one of our accounts,and am now 20% higher than the person he was using before us,he got a little distant at first,and mentioned that he always keeps a few contractors in the wings if the current one does not measure up.I told him to do what ever he thinks he has to do,and I really value him as a customer,and understand ,but my costs went up and I need to cover my costs also.That was over a month ago,and I still have him as a customer,and the slightly negative attitiude over the price increase has gone away,and we are on good terms again.If you do great work and over deliver customers will not want to lose you,and if they drop you for someone cheaper,they weren't worth investing yourself in as a long term business relationship.Tell them calmly and dont let them smell fear or nervousness,it will inspire them to try and rattle you to keep your price where it is, especially if they think you really need the money(as most of us do!)if you act like the price increase is something you cant avoid,and this is your new price they will most likely accept it,and if not see ya!
Good Luck
Jim