Before you start with online betting, it's very smart to gather som info about the "opposition", i.e the bookmaker. It's a good advice to follow this little checklist before placing your money in the bookmakers account:

Reliability/Credibility.

Is it a recognised bookie ? A well known name, with a proven track record ? In other words: Is your money safe ? Check for words on the street, and among other punters. Is the bookie unreliable, or very late with his payouts, etc, then have a look elsewhere. Listen to the grapevine, and pay attention to what others have to say about the bookie.

Methods of payment.

The bookie should have several methods of payouts, not only by bank check, but also by credit cards. I find it most convenient using a credit card in all my transactions with bookies. It may be wise to open a new VISA account, separated from your salary-account.

Rules.

It's a must to know the rules in every game you play, and it's the same story here. Read the rules, and send an email and ask the bookie if you are unsure about something. Avoid bookies who do not accept singles. If the bookie only accepts trebles or doubles, find another. A few years ago, SSP only accepted 4 match parleys. Effectively, this is like telling the punter: " you can have a bet with us, but we will win in 99 of 100 bets". Luckily, competition in the bookmaking market has grown tougher, and as a result, most bookies these days offer singles on every game and in every sport.

Taxes.

Private bookmakers pay taxes to the country they operate in. In some countries, the bookie pay the punter's tax aswell as it's own taxes. When betting with online bookmakers, you should not accept taxes on your winnings. In England, the law states that punters must pay a tax of 9% on their winnings. Do not accept to pay taxes on your winnings, unless you are situated in Britain and with local bookmakers. (abide the law !!!). Online bookmakers do not charge taxes on a punters winnings.

Prices. 

What kind of odds is your bookie offering ? How is the odds compared to what others offer on the same event ? Does the bookie change the odds a lot, and is the bookie presenting his prices for events early in the week ? Even though the bookies do alter their odds (often done on popular events, such as big soccergames, formula1, Championships, etc), you should get the odds which was offered at the time you placed your bet. If the bookie alter the odds on your event after you've placed your bet, and your odds changes as a result of this, go find another bookie. The bookie's behaviour is then nothing but unacceptable and he's not worthy an honest customer like yourself. Normally this is not a problem, but have been known to occur from time to time.

Maximum winnings.

Check if your bookie has got a very low maximum winnings.  For the average punter, this is not important, but for the tough guys, who bet 10 grand rather than a tenner now and then, this can be of some importance.
Minimum deposits
Hopefully you will only have to deposit your hard-earned cash once, but it can be worth having a look at the bookies' rules regarding minimum deposits. Some bookies demand a fortune in deposits. Stay away from these vultures !

Events offered.

The worst thing a bookie can do, is not offering lousy prices. Infact, the worst thing is not offering you the opportunity to bet on an event. Some bookies only offer odds on the top leagues, and never bother about english 2. or 3. division, or Italian Serie B. Some don't even bother with Norwegian 1.division during the summer break in European leagues. It's a major "pre" if your bookie offer as many events as possible, not only soccer, but all kinds of sports. The clever bookie knows, ofcourse, that when many events are offered, the chances are that punters will bet on more events than they originally had in mind, thus increasing the bookies chance of profit. Private bookies do generally offer most events, while the national bookmakers in Norway, Sweden and Denmark only pick a handful of all matches available, and often exclude big favourites in their event lists. Centrebet and Interwetten are bookies who cover a large number of events every week. Do not accept bookies who only cover a few sports, and bookies who do not offer you the chance to bet on lower divisions, or who often exclude big favourites from their event list.