1), inspect your glassware.  Most manufactured glass has small bubbles and very shallow (surface) scratches in it.  These are not normally of concern;  they're at least part of the reason filter flasks are made thicker than ordinary ones.  However, if you see any deep scoring, star cracks, or any cracking at all in the flask, you should replace it immediately. 

2)  Use smaller flasks.  Avoid using any container larger than 1 liter (1,000 mL) for vacuum applications, unless it was specially engineered for the purpose.  With other factors held equal, smaller containers are less likely to implode.