Yes, you have to be careful of annoying your full paying clients and if you make a habit of doing'flash' sales, clients will just wait until one is advertised before booking.
If you have large gaps to fill then go for a big package at a high price. Include upgrades to luxury levels for the normal price perhaps.
I'm always surprised at how expensive I can make a package and it still books well! The £40 to £60 price bracket works well for me and I would do a 1.5 - 2 hour session for this. As long as you make it sound like a promotion people will go for it. It doesn't have to be a big price reduction either. Even if I just take £3 off the standard price, they book it. Perception is everything!