Planning a puzzle giveaway--looking for input

So, I've seen all these puzzle giveaway things floating around, and I like the idea. I'd like to make one, and I'm looking for advice from people with experience with them (from either end).

Often, they involve strings of letters and numbers. The first place my mind goes is usually cryptography and ciphers and the like. This, however, often turns out to be entirely wrong. I was considering making one that actually did involve that sort of thing, but I'm not sure if that would be too difficult for most, too easy for most, or just fine. The closest I've come to a fully fleshed-out plan is an idea I had for providing clues or riddles with one-word answers. The answers would provide letters (and/or numbers) which would be run through a polyalphabetic substitution cipher using a keyword found in a sixth clue, yielding the giveaway URL. This is by no means a set-in-stone plan, and I may well scrap the whole idea and do something else completely.

One of my concerns is, naturally, what sort of puzzle would be best. I've seen some different types, each with their own merits. I don't expect to come up with anything awfully creative, especially my first time out. I'd also rather not do one that's slanted toward people with a specific sort of skill or knowledge (at least not yet--I might do something for my fellow organic chemistry enthusiasts or something in the future; we'll just have to see). I'd like to see a lot of discussion on this point in particular, as I'm not at all certain what the best course of action would be.

Another concern is the difficulty. I don't want to make a puzzle that anyone can just look at and solve in thirty seconds, but I don't want it to be so insanely difficult that virtually no one can figure it out, either. This seems like one of the trickier aspects of puzzle creation. Finding that spot on the bell curve may just take some practice. I figure it's best to err on the side of being too difficult, as I can always provide hints in that case, whereas there's not much I can do if it's too easy. Slightly related is the fact that I don't want to have any instructions that could potentially be confusing or misleading, so I'm going to have to be careful about that. I'm not sure anyone can necessarily help me with that part unless I included them in the process, so once again experience and judgement will have to guide me.

I'm also uncertain about how long the giveaway should run. Firstly, assuming I use the typical method of getting the URL and then manipulating, concealing, or otherwise making it into a puzzle, I will need to allow myself additional time to develop the puzzle around the URL once the giveaway has been made. Depending on the type of puzzle I use, this could take anywhere from a few minutes to several hours, or even a few days if I'm having a lot of trouble coming up with clues and procrastinating severely. I plan on making sure I have adequate time and announcing the puzzle once a predetermined time until the giveaway ends is reached. This brings me to the second issue with the length of the giveaway, namely: how long should it run once it's announced? I want to make sure people have ample opportunity to see it, as well as adequate time to solve it, but I've seen some complaints from some corners about people using things like Yahoo Answers to get puzzle solutions. Ideally, I would be able to devise a puzzle that would be impervious to such underhanded methods, and I've actually seen at least one that I think would, in fact, stymie any such attempts, though I won't be doing one even remotely like it, so that's not much help, really. I'm very interested in hearing people's thoughts on this. Is people cheating their way into puzzle giveaways that big of a problem? If not, then I should focus on allowing enough time to give everyone a fair chance. If it is, then I should try to limit the time to prevent that kind of abuse. I don't even know how to best err on the side of caution with this one, so any advice (especially from experienced puzzlers and puzzlees[?]) would be most welcome.

I think that concludes the issues I'm uncertain of surrounding puzzle giveaways. I look forward to hearing back from you.