In some situations, it's actually acceptable to say "Hey you wanna dance" or (to be more DJish) "Come on, let's dance." Those are mainly the more formal dances, such as swing/salsa parties, but I think they can work in wilder situations also. But you need to use judgment.
In horish-type dances or dances where it's difficult to communicate verbally, it's often best to
1) either wait for an IOI then just go after her and start grinding or
2) grab her by the forearm/hand and start dancing with her from the front
Sometimes I also go for the more wussy way of doing it and I'll get close to her and just try to dance with her first without actually touching. That works on some occasions, too.
I dont her to get bored after a song or two
This is actually a problem for me. I really think dances (of all kinds) were not designed for a single couple to stay together for very long. Even if you're a really great dancer, it's hard to keep being interesting all night long (altho a girl I took to a swing dance lately seemed to be fine dancing with me for 2+ hrs). What they said at Fastseduction.com is to REMEMBER WHAT YOU'RE TRYING TO DO. In many circumstances, you're not just trying to bust out the moves on the dance floor. If you dance with a girl and she likes you, stay with her for a little while but after several songs: change venue, excuse yourself and come back, start making out with her on the dance floor, or just LEAVE THE DANCE AND BRING HER WITH YOU TO ISOLATE HER. If you're trying to score with a girl, the dance was a way to meet with her. If you're trying to sex her, you need to isolate her and sex her. If you're trying to get to know her in some meaningful (LTR-type) way, you need to isolate her and get to know her in other ways.
There's only so far you can get with dance-floor kino (most of the time).