When you touch an item on a store shelf, you are much more likely to buy it, according to a study by Wolf, Arkes, & Muhanna (2008). This is the reason why you are encouraged to "try before you buy" or "take a test drive" as soon as possible.
Once you touch something, it registers into your brain, and you connect with it emotionally (even without realizing it). When we connect with an item emotionally, we unconciously claim it as ours.
The study also found that the longer that people touched an auction item, the higher they bidded for it. People that held on to the item for a short period of time bid an average of $2.44 for it, while people that held on to the item for a longer period of time bid and average of $3.91 for it - an increase of 60.2%!
And true to human behavior, the more competition there was for an item, the more people wanted it.
So if you want to curb your spending, don't touch anything on the shelf!
Wolf, J.R., Arkes, H.R., & Muhanna, W.A. (2008). The power of touch: An examination of the effect of physical contact on the valuation of objects. Judgment and Decision Making 3(6): 476-482.