It's the age-old adage, "Slow and steady wins the race"
Start with minor romance tasks like @Marinuttonen mentioned. Only Flirt at first. MAYBE compliment looks... nothing more than that for a while. It helps to have friendship up high. If friendship is low, then forget romance. Work on getting friendship up first.
There are tricks and exploits you can make use of if you REALLY can't get it done. Quite a few of them actually... The biggest ones...
* Get a romance bucket. The award from getting gold on a date. Enable the emotional aura and instant flirty.
* Paint a flirty painting and enable it.
* Use the fashion photographer studio to take a picture of a flirty sim. The photo will have the "Flirty" buff when enabled.
* Create yourself a "Home" club. Basically a club with all home activities, leave it running all the time until you get tons of points, then put points in all the Flirty bonuses.
* There's a flirty NAP in ECO Lifestyles. (I wouldn't recommend that one, NAPs are all horribly broken)
* Lot traits, Create a sexy nightclub and give it one of the romance lot traits. Take your sims there.
* Aren't there "flirty" cookies?
* Flirty TV shows, books, Valentines day... etc etc