If the item you're after is sold by Amazon you can keep track of its price on there via
http://camelcamelcamel.com/ (must give credit to bigboss for bringing it to my attention)
If you're planning to buy headphones be very wary of websites selling fakes, a quick search on these forums will give you the information you need.
For CD/DVD/Blu-ray purchases I always use billybargain.co.uk
As with buying anything online check for cashback deals at the likes of Quidco or Topcashback