I have had countless problems with pastel fixatives and I have totally ruined many a good painting by using too much of the stuff.
The first solution here is to use as little as you can get away with - and never fix your final layer. Just be very careful with the painting until you can get it framed behind glass...
However, if like me - (and as suggested in another tip by Jennifer Blenkinsopp) - you like to work in layers; you will definitely not be able to completely avoid using pastel fixatives.
My advise here is to simply go for the very highest quality product that you can lay your hands on... For one thing, the price of a very high quality pastel fixative will put you off using too much of it!! And you will stand much less chance of getting any very undesirable "frosting" effects if you do accidentally use too much.
My fixative of choice is the one made for pastels by Senellier. (Senellier also do a very similar fixative which has been formulated especially for charcoal drawings. I find that it also works very well with pastels but it does seem to be a bit lighter - and so you really need to use more of it when working with pastels...)
I can not get hold of Senellier fixative in the Philippines but I find that the museum grade varnish, satin, produced by "Golden" suites my purposes quite well. The only real problem that I have with the Golden varnish is that it takes quite a while to dry.
But then again, that usually gives me a good excuse to go out for a beer!
I buy my Golden varnish at Diovir's in Santa Cruz, Manila
I wouldn't touch any of the other so-called fixatives that you can buy in National Bookstore, with a bargepole!!