I have seen users (including me) who get frustrating registration page and eventually decide to quit the site. (To be frank few of my friends left registration due to annoying captca verification) As I am in a process of designing a kind of social site for which user needs to login to perform certain action. I don't want my users to quit site just because it is either too complex or require more information. The other side of story is that I do need more information from user. Now my question is that what is the possible details I can keep on my registration page to make it comfortable to the user to easily jump to the main site and then how can I capture the remaning information?
My general tips would be:
Question whether you really need all the information you're asking for.
Ask for as little information up front as possible. Not the minimum that your site needs to run as a business - but the minimum that the user needs to continue at this point. You can collect the rest later on.
Explain why you need the information to the user. Don't just say "E-mail", explain with "So we can let you know when your purchase ships".
Allow the user to enter subsidiary registration information later on. Look to sites like LinkedIn that nudge users with "Profile 80% complete" and other messages.
If you really, really cannot avoid a long form all the usual long-form advice about breaking it into self-contained chunks, showing status/progress, etc. apply.
Investigate options like logging in with facebook / twitter / linkedin / google+ / openid / mozilla persona. But please test them with your users first. I have regularly seen these cause problems as well as help (especially open ID - which I've never seen help overall). The general advice would be to only offer options that are related to your product (e.g. Tool for handingly social media followers? Twitter login a no-brainer). But test early and test often.
Do you need registration at all? It's a question that sometimes needs to be asked. Again - do test.
First and foremost:
Don't require registration in a first place. OpenID is one of the ways to avoid registration.
If no registration is not an option, require only the most basic fields you really need to register someone. In 99% of cases, the only field which should be required is the e-mail address. You may put a password field too to avoid perturbing users' habits, but it's not mandatory.
You mentioned that you "need more information". I don't know your particular case, but in general, when a website requires more information, it's a bad sign. E-mail. Password. That's all those websites need. No names. No addresses. No phone numbers. Don't ask me my pet's name or the age of my girlfriend or if I traveled a lot in 1998 or if I'm homosexual. It's not your business.
Asking additional info is the worst thing you can do during registration (especially by making the fields mandatory). If some social website asks me my phone number, I won't give it. Never. Because it's my phone number, because I'm not convinced that this website can't live any longer without knowing the phone number, and because I don't even know how this information will be used. If the phone number is required, I'll either leave the website, or type something like "12345".
If, eventually, some particular features of your website require this additional information, then make those features unavailable just after the registration, and invite persons to enter this information to access those features.
If you're sending no-spam text messages to persons when their friends try to reach them, then there would be persons with no phone numbers who will never receive those notifications, and persons who made an additional step by entering their phone numbers after registration who will be notified.
If you're making an e-commerce website where goods may be sent by mail, you can hardly send anything without knowing a mailing address. This doesn't mean that you should force persons to give their address during registration. Why do you think they have time? Why do you think they will order something right now? Postpone this step until you really need their address, i.e. when they are actually purchasing something.
Ask: Is it really worth it?
If people are leaving your website without registering, that's a big red flag to me that they either don't really need to use your site - or don't understand the value of your website. If the value were otherwise very clear, then it'd be worth the effort of providing the information you so dearly need to provide them the promised service/experience.
The funny thing about human beings is that we're actually VERY adaptable creatures. If there is something we REALLY want, we'll jump through hoops a hundred times - or give up after we're tired of trying/failing too many times - or find other better options.
My suggestion is to do some usability interviews to find out what people think the website can do for them just from looking at the "logged out" experience. Then have them go through the registration process and see if they think it was actually worth the effort.
Worse comes to worse, you can always pay people for their time & trouble to register if you can make that kind of investment ;)