The best way, if it’s possible for you, is to use an ATM in the country you are visiting. If your bank does not charge an outrageous fee for this, you will get a rate that’s pretty close to the interbank rate.
 
Another possibility is to use credit cards if possible, if the merchant does not charge an extra fee for credit cards.

Generally, travelers checks get bad rates.  I think ATM/Debit cards are the big thing now.  Most ATMs charge 1%, and you get about the best rate possible.  You can get descent rates for cash, but you probably don’t want to bring that much cash with you. 

i found, unexpectingly, that post offices are the best place to do that!  They dont charge  you!

ATM’s/minibanks/Cashpoints/ whatever they’re called where you’ll be visiting, are best. I’ve been using them every week for over 4 years in Europe. It might just be Norway, Denmark, and Sweden, but if I use an ATM during local banking hours, I generally don’t get charged a fee; after-hours, and ATM fee is usually added on. Re: credit cards, last year, Mastercard started adding 1% “international conversion fee” to the price for purchases, but that could just be up to my bank. From what I’ve read, 1 to 2% is typical for credit cards; I don’t know of any that still do conversion for free.

I totally agree about travellers cheques. You’re just as well to stash $100-$200 cash somewhere as emergency money in case the ATM network suddenly goes down, or power outages occur, or the ATM runs out of money (that seemed to be a problem for me in Switzerland). You can convert at post offices, Travelex or other reputable exhange windows, or at major banks. Fees will vary. I was surprised this week when I converted Hungarian Forints to Danish Kroner at the airport, and Travelex only charged me a 30 DKK fee (about $5.20), but they might have soaked me on the rate.

I agree with previous posters that using ATMs and credit cards is a good idea. My US bank and credit card comapany have been heavy-handed with fees lately, though. I’m charged about $5 per withdrawal, even though my bank and those from whom I withdraw money from ATMs are part of the Cirrus system.
Something fairly recent has been a charge on my credit card identified as “Finance charge foreign trans.”
As far as I’m concerned, these extra charges are simply legalized theft on the part of large banking systems (Obviously, any calculation of exchange rates is done by a computer, and should cost in the neighborhood of a penny).

As people have said, ATMs are usually best. Some U.S. banks are partnered with European banks and will waive ATM fees for them. Check with your bank. Same for credit cards, which often are not such good deals overseas.