I like sending within Canada, if and when I get a response to a friendly email or an interested party comes forth. I can send a bunch of coins, paying Canada Post only a dollar or two. That is what I call a fun and inexpensive swap.
As far as the strategy... "If you build it, they will come." I try to build up my swap list. In my travels or at home, I stop by a bank or dealer and pick up a roll of something interesting and UNC, 50 cent pieces, Sacagawea/Presidential dollars (Washington, Jefferson, Lincoln) €1, €2, Kennedy half dollars, etc. Having a few uncirculated positions always strengthens the swap list and gives people a base for making a couple of selections.
When it comes to silver, I latch on to one or two members with a huge inventory and offer them my silver finds via Private Message. You'd be surprised how well that works and without getting hundreds of rookie offers. I believe this is called networking or building rapport.
For me, Numista is a tool used to build a base of contacts and then not need Numista or rely on Numista for new swaps. Once you make a few friends and build some trust, your time ends up being filled looking for something that might interest them. They, in return, look for coins that might interest you. Next thing I know, I have 14 ratings and am looking to the unavailable button because I'm in the process of assembling 4 to 5 private swaps.
That is how I do it.