If you want proof that you had item "A" at time "B" then your best bet is simply to hire a local lawyer. You send them your copyrighted material and they take receipt of it and keep it in a safe location. This covers off a number of points:
- Whomever you are using to prove your copyright needs to be impartial. Family and friends don't count.
- You'll want whomever keeps your copyrighted material to ensure that it doesn't get lost, burnt, etc. Good lawyers who deal with this kind of thing will have this in place.
There are also online providers for this kind of thing. UKCS provides this kind of service and I am sure there are other national providers.
If you want it done properly you'll need to pay out cash for this.
On the flip side, if you do find that someone is using your copyrighted material then, before you engage with them, get their documents/web-site/whatever notorised by a solicitor/lawyer. This will provide independent evidence that you were having your copyright infringed in the event that they then change their documents/web-site/whatever in order to avoid further action.
Finally: mailing stuff to yourself is basically worthless. If you want it done properly involve a lawyer.
You don't need to do this in every country for which you want to enforce copyright. The reason you use a lawyer is to have someone impartial who will verify your copyright claim and provide evidence to substantiate it. Obviously, you want to choose a lawyer from countries with strong legal systems (US, UK, Canada, etc) as these will hold more weight than countries where corruption is more of an issue (Nigeria, etc).