This seems to be one of the biggest issues on the site for newer members.
I remember when I first came to the site I used to advertise my boxes not to purposely be annoying but because I thought it was acceptable. Alas, I found out very quickly that advertising was a big no-no here and I stopped right away.
However, over my time here I have discovered acceptable forms of advertising that do not cause an annoyance to other members and yet are still very effective. I will discuss them here.
1) The Critiques/WIP Threads
This is a very beneficial way to advertise your box because not only do more people get to see your work, they are also more apt to help and critique you so you can improve. All you need to do is create a New Thread in the Critiques / Works in Progress section of the forum and then insert whatever you have done on your box into the thread. It is fine if the box is not finished because this part of the forum is for works in progress.
As an added bonus, it is acceptable to place a link to your finished box once you upload it to the site.
2) Your Signature
Probably the most effective way of advertising your boxes, your signature is a great resource and should not be wasted. There are many different styles and forms of signatures but you should only use the ones that are suitable for you. You can create a signature by going to the USER CP and then to Edit Signature.
There are subtle signatures such as mine: StevenCho's Profile | Latest Box | Latest Masterworks
A little more flashy signatures such as this one by sd1833:
My Author Page | HALL OF FAME PLEASE (Click It!) | FFXIII by Aryon
Or artistic ones such as the one used by YoshiStar:
All of these are perfectly acceptable and are with in reason of not being to much of a distraction when people are reading through the forums.
How you create these is very simple. They are all done using links. How you create one depends on if you are using images or words. If you are using words such as mine for example, all you do is type in the text you want in your signature, highlight the text you want to become the link, click on the Insert Link button (the one with the world) and paste the URL. For images, you would insert an image like you normally would and then you highlight then entire image code and click on the Insert Link button and paste the URL.
3) Instant Messaging
If you are going to want to become a strong part of the VGBA community, I would highly recommend adding some of the members here on IM. Some great conversations take place off the site and this is a great place to show off your work. Because it is a little less formal, most people are willing to accept links to your boxes when placed over IM. This is all within reason of course, spamming links to your boxes everywhere over IM would not be considered acceptable and most would not respond. However, most people will reply when honestly asked by a fellow member to critique and view their work.
These are just some of the methods I have discovered and use. I know there may be more so if any of you can think of some others just leave a comment and I will add it to the list.