Isn't all DJing the same? Not really.
If you've been hired for a special event or small pub event, you are purely there for the audience. Unlike a club/bar where people are visiting to hear and dance to a specific type of music, this audience is primarily there for some other purpose (for instance a hen night or just a Friday night out drinking in a pub). The audience is also likely to be a lot more diverse than if you were DJing in a "exclusive" bar or night club. In many ways this makes your job a lot harder, because you have to try to avoid playing a fixed style of music especially your own favourite tunes.
Variation is the key, so by playing a lot of commercial pop or similar music, you maximise the chances of pleasing the maximum number of people in the venue. It's best if you try and "read" the crowd, and try to work out what sort of music they like and dislike. Also, try to group the similar types of music together so you play several different "sets" to help build up an atmosphere. This is important because many guests don't get up and dance immediately and might wait for others to dance first. If the dance floor fills you can carry on playing more of the same style of music, and if the dance floor starts to empty at any point you can simply start playing a different "set". As most people won't want to dance all night long, they can buy their drinks from the bar and chat to friends until another you play more music that they like and want to dance to.
Music is of course just one aspect - you could easily improve you "business proposition" by including party games, karaoke and the like. For weddings you could also include extras such as videotaping the reception party. You can also help build up the party atmosphere by using specialist lighting rigs and smoke machines (though be careful that the smoke machine doesn't accidentally set off any fire alarms close by!)
Unfortunately, "proper" karaoke machines using video CDs are very expensive, and likely to cost too much for beginner to purchase. There are cheaper alternatives though, as you can often find kareokee CDs in most record shops. However, it would be quite expensive to buy all you favourite/popular tunes twice (as normal CDs and as instrumental kareokee versions). Another alternative is to use relatively cheap DSPs (Digital signal processors) and/or Analogue Equalizers on your mixer to help reduce the level of vocals being played. Although this method isn't perfect, the fact that the person participating will effectively have a backing singer (the original vocal on the CD), could help to boost peoples confidence and encourage more people to sing than the traditional purely instrumental kareokee video CDs.
If you decide this style of DJing suits you (or is the most profitable or easiest way to find work), you are likely to find that you will be traveling between many different venues, where most have not got a dedicated area setup for DJs and their equipment. Therefore the best equipment to use might not be the same as what a typical club/bar DJ would use (where the venue is designed to have space for you and all of your DJ equipment). More info can be found in the equipment section. It is particularly important to ensure there is an adequate power supply available. I would recommend you also try and carry some backup equipment, in case your main equipment fails, and possibly some spare fuses in case one blows. To help protect your equipment it can be advisable to use a voltage regulating power splitter. Other items might also be needed but it really depends on your equipment.
Finally, and this is probably one of the most important points - to be a very successful mobile/special event DJ you need to use your personality to entertain and encourage the audience to participate in what you are offering. The best mobile DJs are in fact "Party Hosts" with great personality and stage presence who are confident in entertaining the audience and encouraging everybody to join in on the fun! If you rely on just music, then anyone else could potentially come along and offer the same deal to customers at a cheaper price. If you have that "something extra" then you'll be a big hit, and hopefully be recommended to friends, eventually creating a "pyramid" of recommendations, ensuring you career is a long term success.
Once established as a mobile DJ, another small thing you can do is to make friends with another DJ who plays similar gigs to yourself. Not only can you share ideas, but when you are already booked, you can pass any leads to your DJ friend. However, this will obviously only be beneficial if the friend DJ gives you reciprocal leads, so you do have to trust one another.