1) Create a budget.
What is the ideal amount you would like to spend? What can your family afford above/below that amount? Do you have money set aside to pay all at once, or will you be trying to take advantage of a payment plan (many companies offer these)? Nailing this down first will help you make tough decisions as you go, rather than getting your heart set on something only to realize you can't make it happen after all.
2) Create a master list.
Begin with considering which method (or methods!) of schooling best fit with your family. Think about what your mission is a homeschooling parent and what your goals are - as a family and for each particular child. Then list any requirements in your state for the ages of your children. Once you have these parameters in place, you can list the subjects you will be covering, leaving space to fill in which materials you will use as you find them.
3) Research your options.
The internet is the best shopping tool on the planet, hands down. Not only can you look over samples and lists at homeschool curriculum companies, you can also read the reviews of other parents who have used those materials and connect with other homeschooling parents to glean their perspectives on different items. Talk with the other homeschooling families in your area; ask what they are using, what they have used, and what their thoughts are concerning your curriculum quest.
4) Attend used curriculum sales and homeschool conventions.
Wether or not you purchase anything at these gatherings, you will benefit from the time you spend. It is one thing to look at an advertised book or system on your computer screen, but it is quite another to hold it in your hands, flip through all the pages and speak with the parent (and sometimes student) who used it. First-hand accounts like this and the option to read over the material helps a great deal in the decision-making process.
5) Fill in the blanks.
Once you get this far, you will have most of the items on your list filled. I usually don't make any purchases online (though I may have a saved 'shopping cart' waiting to click 'check out') until I've been to the events in step four. Then, if I have a few things I didn't find or still need, I can take advantage of bulk rates or discounted shipping when I add to my order. I may beg or borrow materials from a friend or put together lessons from a free resource online.
What have you found to be your best resource for researching curriculum?