I've been recently offered a couple of Postdoc positions, and I was wondering how you as professors would prefer to be informed if a candidate would rather work with another institution.
I know we are professionals, and these things are to be expected, and probably the professor has another candidates to choose from, but I just want to be as respectful and professional myself as possible, so how would you turn down an offer, or would like to be turned down.
Keep it short and honest.
For example,
Dear So and So, thank you for your offer for a postdoc at Blah. It's an attractive offer, but I've decided to take a position at Foo-Bar instead. The work you're doing over at Blah is very interesting and I look forward to interacting with you in the future.
Of course if you have more than one rejection to send out PLEASE PLEASE don't do it generically. Tailor it to each place individually. It takes a little more time, but it's a fair exchange given that they took the time to give you an offer.
I agree with the previous answers: short, honest, polite. I'll add: prompt. As soon as you accept an offer, let the other institutions know immediately. That will let them move on with their search.
Just send a short but polite note telling them something like: you appreciate the offer but have decided to accept a different one. Maybe tell them (especially if it's true!) that you respect their research, hope to see them at a conference or visit their institution in the future, or something nice along those lines. Don't overthink it, just be polite.
First, I really like Suresh's answer. Short, straight, polite, and mentioning you would like to work with them in the future.
When I did my post-doc, I had the same problem: I applied for more positions than I could accept 8^)
Turning down the offers I could not accept was along Suresh's lines, but with one major point added: I did not send emails/write letters, I called the professors and talked to them personally.
And yes, I met all of them again during my post-doc. The world of research is very small.
one additional thought: If the offer you reject is a good one (i.e. a lab you can see yourself work in), I really believe it is important to call the PI you turn down, confirm your interest in the research done in the lab, and make clear that it was a really tough decision for you to make. Being in similar situation coming out of my PhD, I ended up choosing lab A over lab B, and after a few months, I realized PI of lab A was a nightmare, so I contacted lab B again and had a productive postdoc there.
