But when you're giving to members of your own generation, or to ones above yours, it starts to sometimes feel a little inappropriate. In these cases, I think it's better to try to actually buy something or to resort to a gift certificate that doesn't feel as crass as cash. How much you should give depends on your budget, and if you're giving to a child, how old they are. As for giving to adults, give whatever you can afford. They should graciously accept anything that comes with your good wishes.
Big spenders ought to keep the tax laws in mind, however.
Keep in mind, though, that to avoid paying taxes, you not only have to write the checks by December 31 to avoid, but the recipients have to deposit them by that date as well. As an alternative, you can use certified checks, which are treated as given in the year they are delivered. They'll rarely pick out the thing they really want because they'll worry it's too expensive. As the giver, it's up to you to decide how much you're willing to spend.