getting an outsider to do work on my site

I am in the process of hiring someone to install a script for me. What is the best way to go about restricting access on my account?

The best way is to add an FTP user in your cPanel and give them access to the directory that you want them to install the script in, and nothing else.

Aside from that, it is not uncommon to give out passwords to people that are working on your site.

Typically, if they are working for you, and you maintain a mutually beneficial business relationship, there is no problem. If that person is in business to install scripts, they will not want to "screw" their clients. If they do this, then, they go out of business.

Just an insight for you, definitely not a rule of thumb.

Best thing to do is make sure you do your research on the person doing the work for you. There is no reason a reputable service provider would do anything malicious.

In all cases, change your password or delete the FTP account for this user if they are only temporary help and you are not sure of their legitimacy.

I have personally changed the password temporarily to grant access for support, then changed the password again when the work was complete.

Matt is right though. There really isn't any reason for anyone to be malicious unless they are just into that kind of anarchy. :)
Follow his advice, get some information about who you are hiring and ask for references. (You can google them too for reviews!)

I like to ask people I respect for recommendations on services or products also.