Adding your signature to a painting is like adding a stamp to it that reads "finished". It's a sign that you're satisfied with the painting and no longer consider it a work in progress.
Is It Really Necessary to Sign a Painting?
It's not a legal requirement, but if you don't add your name to a painting, how will anyone know who the artist is? You may argue that you've a very familiar style that people will recognize, but what if it's the first time someone's encountered your work? How will they find out who the artist is then? If it's hanging in a gallery it'll have a label with your name on it, but what if it's in the lounge of someone who's bought a painting and they can't remember who the artist was? Think about the works by famous artists which are 'rediscovered' every now and then; is this a fate you want to risk for your paintings?
What Should My Signature Look Like?
The most important thing is that people must be able to read it. An illegible signature isn't a sign that you're extremely creative and it doesn't add a level of intrigue to the painting. You're the artist, so let it be known. But at the same time, don't make it look like you're using a stamp. You don't have to sign your whole name on the front of the painting, you could just put your initials but it's wise to put your full name on the back of the painting. The same applies if you use a symbol or monograph; people have to have some way of knowing what it stands for.
Should I Put a Date With My Signature?
I believe you should date a painting, though it needn't be next to your signature on the front. The reason: when you first start painting you'll probably be able to keep track of when you painted a particular painting, but wait until you've several years' worth of paintings, then you'll simply be unable to remember and will have to guess. Serious collectors and galleries like be able to see how a painter's work has developed over the years, so get into the habit of dating your work now. You don't have to put the date on the front of your painting, but could write it on the back (though once it's framed you may not be able to see it). Or put only the year on the front and the month and year you completed it on the back.
I don't buy the argument that putting a date on a painting limits your potential to sell it. Art isn't like food, a product with a sell-buy date. If buyers only wanted the newest and latest work, then how come there's an auction market for contemporary paintings? And if anyone asks why a painting from a few years back hasn't sold, tell them you'd kept it in your personal collection until now because you regard it as a key work.
Where Do I Put My Signature?
It's up to you, though traditionally a signature is put towards one of the bottom corners. A signature should be an integral part of a painting and not detract from the painting. Be consistent about where you put your signature as then when someone next encounters a painting they think by your, they know exactly where to look to check.
What Should I Use to Sign a Painting?
Use whatever you've created the painting in, whether it's pastel, watercolor, whatever. Try to remember to sign the work before you clean your brushes and palette for the last time from a particular painting so you've got a suitable color to hand that will blend in with the work. (I do it with a thin rigger brush.) Having your signature 'match' the painting, rather than it looking like a later addition, also makes it less likely that someone will question the authenticity of the work at some future date (most likely after you're dead and your paintings have increased in value enormously). Avoid adding your signature on top of a layer of varnish as it'll look like you forgot to do it in time (and if you must, keep it small and rather put your full signature on the back).
Should You Sign a Painting with Your Maiden Name or Married Name?
If you change your name when you get married, how should you sign your paintings? Should you continue using the name you had been, your maiden name, or should you change to your new, married name? Ultimately, it's a matter of individual preference.
If an artist is already known professionally by a maiden name, it wouldn't make sense to change it because you'd have to remarket yourself. Or if both partners are artists, then sometimes people prefer to have different names to avoid comparison. Using a maiden name certainly solves any problem if a divorce later happens, but it's hard to say to a new partner because it implies a lack of belief in a relationship, which it isn't the issue it's tied into at all. Your personal identity as an artist may be strongly tied into the name you've had since birth. There is no correct way or choice when it comes to signing a painting with your maiden name or not, it's an individual choice.
What About Limited Edition Prints?
When you create a limited edition print, always indicate how many prints were made and the number of that particular print, for example 3/25 (the third print of a total of twenty-five), as well as signing it.