If you have an existing site and want to add in extra functionality, then there are a few things that you need to consider.
Firstly you should try and advise your website programmer of all the functionality you are likely to want to add in in the near future as often changes are easier if done all at the same time, which means you should be able to save some money.
Once you know exactly what you want to to add in and you are confident that the programmer understands, then make sure that a complete back up of the website is taken. This way if anything goes wrong, you can quickly revert back to the original site. Ask the programmer if they are able to work on a local copy of the site and set the changes live when it is completed rather than working on a live site as this can cause a host of problems.