There is also a link on the bjcp site for upcoming competitions. It helps to plan ahead and have beers brewed so that they are within a window of when they are best to drink.
In my experience, I find the bigger BJCP comps to provide the better, more consistent feedback. Sometimes smaller comps are more like "beer tasting events" than true BJCP competitions - or, there are fewer qualified judges and your beer might be judged based on if the judge likes the way it tastes, as opposed to meeting style criteria. Not always - but overall, my better feedback has come from the bigger, more established comps (500-1000 entries).
It is important to read through and understand the style guidelines somewhat so you are entering beers in the right category. It is not that hard to figure them out - especially if you use a brewing software that shows you the criteria of the category you say your beer is (iBrewmaster does this, and I am sure others do also).
If you find yourself trying to decide if your beer is an American pale ale or and IPA, or an ordinary bitter vs. a premium bitter and you are "on the line" for gravity, bitterness, etc.... I have found that I am usually more successful entering a bigger version of a beer in the lower category.... as opposed to entering a "small" IPA, for instance.
Depending on what your goal is with competitions, one bit of advice I would give is this. If your goal is to get honest feedback to improve your ability to brew beers to style, I think it is necessary to enter the same beer, from same batch, in 3 or even 4 comps about the same time. Then you will get a much broader set of score sheets and you can really see what things come up as consistently good or bad. 1 comp kind of leaves the possibility that your feedback could be a bit hit and miss due to experience of judge, whether they like the style, did it follow a great beer (or a horrible one), random luck, etc. Not that you have to send every beer in to 4 comps. But, for beers that I am really trying to dial in, I send them in 3-4 places.
Overall, I have found that starting to enter comps. in the last year and a half has really helped me improve my beer.
