I suggest dumping the batteries they come with and installing high capacity re-chargeables, like around 2200 mAh. And clean the battery contacts every couple of months - they sit outside, and damp air can cause enough corrosion to make for a poor contact over that time.
I have 3 year old ones from qvc that are still working and I have never done a thing to them. This year I bought these
http://www.qvcuk.com/...cm_scid.KeywordSearch
which give a very pretty amber glow and are cheap as chips.
I bought mine in the £1 shop and have had them for about 4yrs . Never changed the batteries in any of them and they are still working and they look good too.