Simply getting "more visitors" is actually quite easy. Just sign up for one of those spammy link-wheel programs and you'll get hundreds, maybe thousands of visitors. Of course, they'll only stick around a second or two before they move on... but there will be plenty of them.
is getting more visitors who are actually interested in buying what you're offering
. But when somebody says they want more traffic, this is usually the kind of traffic they're talking about.
Since you posted this in the Pay Per Click advertising forum, I assume you've considered PPC? That's an excellent way to bring in more qualified site visitors. It's also a great hedge against the inevitable fluctuations in traffic from organic search. You just never know when the search engines are going to change their algorithms and your organic search traffic takes a nose dive. PPC can easily step in and take up the slack when that happens. (Notice I said "when," not "if.")
You should also look into beefing up your direct referral traffic. That is, traffic that comes directly from links on other websites. Find companies you can partner with, those that offer related (but not directly competing) services, those that serve the same audience you want to target. It's OK to exchange links with these types of sites -- don't let anybody tell you that "all link exchanges are spam" -- or you can offer them a "finders' fee" for qualified referrals they send your way, if that's what it takes to get a link on their site.
You might also take a look at traditional advertising, both on and off line. It can get a bit pricy, and it's true that clickthrough rates for banner ads are relatively low, but it may be an alternative that works for you. Especially if you've got a brand you can promote -- it's unlikely that you'll lose search position for a branded term unless you've done something really black-hat. Those branded search terms can save your cookies when the keyword terms fall away.
Email and direct mail marketing are also possibilities.
The thing is, start thinking about how you'd promote your business if there were no such thing as Google. What would you do to get the word out then? What would you do to get your phones ringing? Do those things, regularly. Don't let yourself get into a position where a change in the organic search algorithm can seriously affect your business. If you're already in that sort of place, do everything in your power to get out of it.
If you're looking for a way to "fix" your site to try to bounce back from the recent Google updates, you might want to take a look at this article from Search Engine Journal: Penguin or Panda? How To Determine Which Google Algorithm Update Impacted Your Website
. And be aware it can take six months (or more) to recover, so you should probably consider other promotional and advertising channels in the meantime.