I am not sure exactly how much money you plan to invest in this, there are a few things to think about.
If you are planning on putting a couple thousand in, you could do a popular method of investing a fixed amount each month. This is fairly safe. For instance:
If you buy at 30 1/2 and for some reason next month it's down to 29 1/2 you can buy more for cheaper. If it's up to 33 1/2 you can still buy more, a growth company isn't going to stop growing.
Or, if you don't have that much money, and you want to sink it all in at one time there's some things to consider.
This stock probably won't go below $27 unless it splits sometime in the future. But that doesn't mean it won't dip down to $27, and you could make more money if you waited for this to happen of course.
That strategy does have a few flaws though. It might not dip down that much, and you could miss out. When you are buying a stock you should hope it keeps going, which means you should think long term on this one. This means you should be willing to take a few dips every now and then if in the end it has a solid growth outlook which, in my opinion, Staples has.
I don't see this baby dipping below 29, if you are worried about a down trend you can always hedge with a few puts. This stock has held up through these last corrections and still showed support in the 29 range, but, if I knew how to predict, I would have had 6 numbers a long time ago and been hanging with Mr. Buffet