To the UK use EMS. It's very quick, and very secure and of course you get tracking numbers.
To make things cheaper and more secure ensure the item is in a fairly large box (not too large, use your common sense) so that it's not too small to get 'lost' .
Next ensure that the item is marked on the docs as having no commercial value and contents are worth less than $50. This way you will avoid paying our personal import duty - which is bloody expensive. <IMG SRC="smilies/ohno.gif" border="0">
If you're posting your end using USPS it will be delivered in the UK by Parcelforce. Parcelforce will change the tracking numbers automatically to their system when the goods enter the UK - so you can still track them.
Using Fedex, UPS, DHL etc. you won't need to worry about that - but these services cost more and I don't think they're as good as EMS for this type of service.
Any further questions PM me. <IMG SRC="smilies/biggrin.gif" border="0">