Some girls manage pretty fine to still retain that first joy of fucking, those who had to learn it on set first are usually better off after the 10th LP scene.
Others lead a private life helping them a lot like Belle Claire for instance - Inga Devil and Timea Bella would also have been good examples for a long time. Some however have a more difficult daily surrounding, the more they work on porn sets the less they gnuinely enjoy their job and the actors they work with.
Lets try a list of good examples over a LP career of 2-3 years. Actresses who still surprise you with a joyful freshness even in their 10th or 20th LP scene:
For me its Arwen God for sure, Susan Ayn and Lola Taylor to a bigger extent.
Some more come sure soon to mind.
At this point maybe Arwen to start with:

Arwen in Prague, 2nd year perfoming for LP, during her 12th DAP scene. (SZ985)
http://www.legalporno.com/watch/30315/a ... n_lp_sz985