First make sure that your back, legs and hips are thoroughly stretched out. Then perform the following:

Bar X 10 reps
add 50-100lbs X 5-10 reps
repeat until you are within 50-100lbs of your first work set.

The amount of weight and reps will vary depending on what your first work set is. Here is an example for someone who plans to lift 315 for his/her first work set:
BarX10
135X5
225X5
315X5

If you were only working towards 225, I would do the following:
BarX10
135X5
185X5
225X5

A more experienced lifter who's work sets are considerably higher may need a different approach. In this case, the lifter will need more total sets to get to his work set. Since more sets are involved, less reps are required per set in order to get properly warmed up. Here is an example for someone working towards a workset with 500

BarX10
135X5
225x3
315X1
405X1
500X5

In summary, it all really comes down to doing enough sets and reps to properly warm up your muscles and slowly acclimate your body to the bar weight of the first working set. I have given you some general guidelines. Now you have to play with it and find out what works best for you.

Hope this helps,
Clint Smith