try using a flavored lube. i like using watermelon flavored H2O lube when i give handjobs and blowjobs.
also, ask her to start giving you a handjob with the lube first. as she's stroking your cock, ask her to put her mouth over the head at the same time.
as she gets use to this, she can start moving her mouth up and down on the head and shaft in the same motion as her hand. my hubby loves when i suck his cock like this because it feels like i'm deepthroating him.