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It would be silly to seal a two sentence thank-you memo in an envelope and formally present it to the recipient. On the other hand, you might come off as rushed or even lazy if you deliver a formal letter carelessly. The best way to give a smaller thank-you note is to put it somewhere he will normally go. Personally handing someone a thank-you note will come off as redundant, so it's best for the recipient to see it when you're not around. For example, if it is a fellow co-worker, you could attach it to his cubicle as soon as you get to work. Another opportunity would be to be to stick it to the recipient's door. Although it's not a likely risk in a professional note, it's a good idea to exercise some discretion with a note. Assume that anyone could potentially read it, especially if you're leaving it in a high-traffic area like the office space. If there is any doubt, it's best to seal your letter in an envelope. Unlike a simple note, which is more informal and spontaneous, giving a letter can involve some ritual and formal courtesy. Feel free to seal it in an envelope and place it in their inbox, or even hand it to them personally. If you give it personally, make sure to bolster the letter with a smile and a firm handshake. In some cases, it may be appropriate to attach something inexpensive (>$10) to the note as a way to better manifest the gratitude. Take care only to do this when it is appropriate however; you don't want to make it look like you're trying to bribe someone.  If you attach a gift, it is best to deliver the letter and gift personally. It's not recommended you personally give a short note to someone personally, but a bigger letter (especially when coming with a gift) may be appropriate depending on the circumstances. If you decide it's best to give it to the recipient directly, you should keep surrounding circumstances in mind. If the recipient is bogged down with work, he might be annoyed by any new distractions. Waiting until a lunch break or a lull in the day's busyness is a good time to hand it over. You would give very mixed messages if you don't personally show the same gratitude you wrote in your letter. This is especially true if you have written the letter purely as a professional gesture. If and when you deliver the letter, you need to make sure your personal behaviour is in line with the meaning of the letter. Be confident, and remember to smile. If you're feeling anxious, take a moment to remember the fact that very few people have felt anything other than gratitude and satisfaction for being thanked for something!
Decide how important and formal the letter is, and deliver it accordingly. Post it somewhere visible. Seal your letter in an envelope. Attach a minor gift if appropriate. Wait until an appropriate, quiet time if you choose to deliver the letter personally. Be polite and gracious while delivering your letter.