Mechanical pressure contact
Technically this is a rebut to Scott Seidman's answer, but not practically.


You could have your contact stamped out of bimetallic material, so that it curled away from the board when hot.  On reflow the solder should (or maybe not) solidify before the contact has finished curling into the board.  So, either a nice preload, or a device that reliably rips itself out of the partially-cooled solder every time.
You could make the contact with an inverted "U".  Solder it on with no preload, then crimp the "U" tighter after soldering.  Presto -- instant preload.  With more elaborate crimping tools, you could use a mostly flat strap.
You could contemplate the cost in engineering hours and grief of either of the above two options, and consider that a cheap surface-mount slide switch is 30 cents in quantity.  Estimate the engineering cost, divide by the anticipated quantity, and compare that to 30 cents...