Kind of depends on how much you're bringing. If it's less than 10-20 thousand francs/dollars/ etc, cash is probably your best bet and lets you choose more precisely what your exchange rate will be when you bring it over- for such amounts and smaller, doing it through a service will probably rape you with a huge spread on the exchange rate. If it's more than that, you might look at xe.com , they
have a rather reputable forex business.. you open an account with them, wire your money in, convert it, and wire it out. they have pretty good spreads and you have finer grain control over the timing of
the trade.

When you do the exchange, you will get a considerably better rate doing it at a bank you have an account with.. several percent narrower spreads.
good luck!