Strategy- How to make a single time visitor into a frequent visitor?
I know subscribing is one of the many options for user engagement and frequent visits by users.
First, you should accept more answers :)
Now, to the point - this may not be the definitive answer, but have you been tracking ANY sort of data for any visitor to the site? i.e. what do they do, where do they go, what do they search etc., etc.,?
You WILL need some data like this to do some analysis to know what patterns exist and what is attracting them the most.
Once you have that you'll be in a better state to even decided what your strategy should be...
...don't be surprised if you have to refer you college statistics book :)
You may then (re)structure the site accordingly so that you target those parts more often. Have easy to access links for subscription or sign up using existing open-id accounts like that on stackexchange, gmail, facebook etc., so that you can be 'connected' to them, so to speak.
Keep the colors pleasing: Try theme-ing on colorschemedesigner.com for example. They can be turn offs in some cases.
You'll basically need a good amount of data to base your decisions on. If you don't have anything as such - you may try trial/error approaches or start tracking for the next 30 days and be more confident about the proposed strategy :)
Hope this helps.
Don't make alerts, but rather get the users to visit you to see if there are any updates. This is the reason for StackExchange not having real-time email notifications when one gets a response.
The other thing is that you should make sure that you have fresh and interesting content whenever the users goes to check up on whether there are any updates. Your ultimate limit to the number of times you can get a user to visit, is the amount of fresh and interesting content you can come up with. When thinking about this, think of content in a very broad way, even the reputation points and badges can be considered a form of content, which may lead users to check up on updates more frequently. Other forms of content are: