I have seasonal allergies and I found an effective method for myself: When I realize the allergies are kicking in (usually after 6 hours of watery eyes and sneezing) I take a claritin and a zyrtec together, as well as spraying my nose with Flonase. Usually this knocks it off and I will keep taking one of the once-a-day meds for a bit to prevent reoccurrence.

Zyrtec and Flonase together is probably the best normal combo and is generally accepted to be ok.

Disclaimers: I'm not a doctor. Combining a nose spray and a pill is generally accepted practice and studied in several peer review studies I've seen. Stacking claritin and zyrtec pills together is not generally accepted practice, so don't do it.