Essentially, you contact club (or VIP) hosts to make your reservations. Some club hosts work for the clubs themselves. Others are independent operators with affiliations at the clubs. Still others work for host companies that charge a fee to get you on a guest list, usher you inside, and/or make sure you're taken care of.
You can contact club hosts directly by googling a club and searching for its hosts; most of them know one another, so if one can't help you (it's not his club), once you make contact, he can turn you on to his counterpart at a club you're interested in.
A few of the bigger "hosted entrance" companies include JackColton.com (which has explanations of the scene and lists of hosts), LavishVegas.com (with typical charges of around $70 for men and $35 for women), and VIPNite.com.
Some tour companies, such as Nite Tours, will take you on pub crawls -- the $89 VIP Pub Crawl fee takes care of the cover charges and expedited entry into 3 top nightclubs, starting at 9:30 p.m. The Party Bus tour ($69) is good for entry to two nightclubs; the $39 Party Pass gets you into one club. Other tours include Ladies Night, Ultimate Bachelor Party, 5-day Pool Pass, and Wild Ride (bachelorette, divorce, and birthday parties).
As usual around here, whatever special treatment you want, you can buy.