What I use for heartworm and general worming prevention for all of my dogs is Ivermectin 1.87%. It is a Horse Paste Wormer (my favorite is the gel). It is an excellent and cheap alternative to the vet's products.  One tube is around $5 and it will last your dog all year.  It has several brand names so just be sure to look that it is the only active ingredient on the label.

Your vet may tell you that it is not safe or that it is not mixed consistently.  The TRUTH is that it is perfectly safe if given the proper dosage, and it is thoroughly mixed when manufactured.

Ivermectin will not kill external parasites such as fleas, ticks and  mosquitos.

HOW TO ADMINISTER IVERMECTIN TO A DOG:
NOTE: A whole tube of horse paste is a enough to treat a 1250 lb. horse!!!  On the tube plunger (pictured left below), there are notches and doses marked in 250 lb increments.  Each notch is enough for 100 lbs of dog (amount pictured on end of yellow below), so you can imagine how little you will need for an IG!  However, 1 notch will not hurt an IG.

STEPS TO ADMINISTER IVERMECTIN PASTE or GEL:
1.  Push 1/2 a notch out of end of tube.
2.  Put the dab on your finger.
3.  Open dog's mouth and wipe it on the roof of mouth.
4.  Hold mouth closed for a few seconds so none gets spit out.
5.  Mark on your calendar the date given and when next dose is due.  Also helpful to mark it on the ivermectin box.

Do your homework to find the right flea and tick prevention for your dog. 
The Italian Greyhound Club of America has some good info about this.  (see link left sidebar)