Know the weight of what you're towing, and what the Towbar is rated to handle.
A lot of FWD cars, the towbar ends up with a rating which is only fractionally above the unloaded weight of your average homemade garden trailer
![Shocked 8O](./images/smilies/icon_eek.gif)
Though that limit goes up substantially when its braked.
To give you an idea, my Caldy's Towbar is rated 450kg unbraked / 1500kg braked - and I have a rear axel to bolt to
Engine power isn't the biggest worry - ultimately unless you have some uber steepness issues and/or your car will be heavily loaded whilst towing, most cars can handle a trailer borne load without hassle - but braking and general control are of concern. If the trailer has more than a certain amount of mass, jack-knifing under braking or cornering is a real risk. (My dad's 3SFE powered ST191 Corona was jack-knifed by a trailer load of dirt... not pleasant!)