Everyone likes something for nothing, and the bonuses casinos offer to attract new players and retain current ones are no exception. That said, it's worth knowing about the withdrawal restrictions that might apply to such promotions.
The most common limitation on withdrawals is the wagering requirement. This entails the player having to wager many times over the value of the bonus (typically around 30 times) before any cash can be withdrawn. The reasoning is pretty straightforward, as the absence of such a requirement would enable players to get the cash and take it immediately. This sort of restriction usually applies to free credits/spins and sometimes cash bonuses and deposit matching too.
To make matters more complicated, some game contributions go towards fulfilling wagering requirements at different rates. For a flexible cash bonus that can be used on any game, some (table games, perhaps) will contribute 10% of bets towards the wagering requirement, while slots will usually count for 100%. This increases the actual amount that has to be wagered in order to meet the requirement.
Game eligibility can be another factor. Some promotions (free spins/credits) are often tied to a specific game, so these aren't affected. But deposit matches and cash bonuses sometimes have a limited number of games that are eligible for inclusion in the offer.
You may be excited to win a large amount and finally meet the wagering requirements, but it's important to keep in mind that many times you will have a withdrawal ceiling on your bonus winnings. For instance, you may have $1000, but the casino may only allow you to withdraw $200 and forfeit the remainder. This is just so they don't lose too much money with their lucrative promos.
The biggest single recommendation is to check terms and conditions carefully. Browse the market. It's rare, but occasionally you might find a cash bonus that doesn't have a playthrough requirement or ceiling on winnings. If you can find something like that, it's a pretty sweet deal.
A more common advantage to browsing the market is that the restrictions on promotions are as important as the juicy part of the deal. It's all very well getting a larger than usual cash bonus (for example, $50 rather than $30) but if the wagering requirement is relatively higher (say, 50 times the cash amount rather than 30 times) that makes it more difficult to finish ahead.
A general tip, if you're just after special offers and aren't fussed about the game type, is to go for slots. Slots have a relatively low edge for casinos, increasing the odds on finishing ahead. (And, when there are different contribution levels, they tend to have a 100% rate).
Compare the pros and cons of a bonus before deciding whether it's what you're after. Fortunately, there are plenty of deals on offer, so just browse the market to find the right bonus for you.