If you build up links too quickly then search engines will penalize you by throwing your site back in the results.This has happend to me many times. Popular ways of link building include directory submissions, blog posts, article submissions, forum posts, link exchanges and obtaining links on websites with the same content as you. If you do this gradually over time at a steady pace then you will see positive results.
I agree with what DrJale said. You shouldn't focus on getting a lot of backlinks in short period of time. It's better to focus on quality. You should consider outsourcing if you don't know or too busy to build those backlinks by yourself.
If you do some research on 'viral marketing' then it is entirely possible to build loads of links quickly and cheaply. It's a very hit and miss science, but if you hit then your site's traffic can literally explode over night. If you are willing to take the risk, you could try some viral marketing strategies for a week or two instead of traditional link building - you might get lucky!
These tips is take from my observation , i think all of us have great effort to link building from directories, social bookmarking sumition, link exchange, and soon. This is right but can be wrong also. Because not all directories, social bookmarking, and blog/web that have link exchange with us, giving us quality backlink that improve seo and PR. Say it isn't much help you with link building if you get nofollow and redirected link, or low rank link. A lot of directory give us bad link that has nofollow and redirected. Some tips to build quality link fast : 1. Link exchange with other blog/web that use put your link on Homepage, this will ensure you get quality link as you think. And based my observation , HP link give much link every new post updated. 2. Blog list / snipped your article feed on other blog. This snipped rss will give your blog/web a link for every your blog article update. 3. Active on forum that allow trackback and use signature, this will give you a link every you post a posting. 4. Quality commenting, use google and seo quake to focus and targeted link building. So you can ensure to link bulding in a high PR page and same topix. If consent i think we can comment as much as possible in one free day. With this tips i build 2000+ perblog link for 1-2 month domain age.
With links, SLOW AND STEADY wins the race. Yes, you can ramp it up, but only once your site's been around for a while. Don't look for shortcuts you will get your fingers burned (aka slapped by Google).
There was a thread with a pretty good guide the other day, but yeah, the methods have been summed up already. Articles, forum posting, directory submission etc.