You can still experience (some of) the thrill
of the first kiss, even if you've been a relationship for months!
All you have to do is both pretend you've never kissed before!
I usually compare all my first kisses to when I once pulled (as they say in the UK) someone in a club. She'd approached me, we'd just talked for hours, and then we'd gone to dance, both brimming with anticipation. First we joined hands, then moved in closer, then her eyes shut and her head tilted back... and slowly (because you have no idea what the other person kisses like), our lips met.
A perfect scenario to enact with your partner.
Maybe put one hand on his or her cheek, and slowly, slowly, come together, lips slightly parted, and kiss tentatively. This really does somehow work, especially since being with the same person you likely kiss each other all the time almost as instinct, without the passion and anticipation of new lovers..
But a first kiss I've found changes the mood, you're not just snogging each other's faces off, but you're doing it as tenderly as possible... time slows down, suddenly you become tingly, feel a bit warm, and squeeze your SO a bit tighter.