Timeshare has been around since the 70s, and many companies are well established and operating legally. However, be careful, there are also many companies who are unscrupulous, claiming to provide incentives.
You should be wary of any company that calls you out of the blue, sending letters and emails, offering you help regarding your timeshare. If you are contacted by companies like these, do not pass on any confidential information.
If you do carry on with such companies, you should never make any investments with them without doing your research first. Of course, you will most likely be put under pressure to buy the product that they are offering, always say that you would like to asleep on it first and do your research before saying yes. If you manage to get any paper work from these companies, always get a timeshare contract specialist to look over them and give you their opinion.
Before signing, always ask about your cancellation rights and make sure you get them in writing. You will always have a 14 day cooling off period, in which you can withdraw from the contract without any kind of penalty. Also, during this cooling off period you should never pass over any payments.