They are usually thrown to help family and friends of the couple get to know each other better before the wedding. They can be formal as well as casual affairs. The decorations of these kinds of parties are very important. Recent years have seen many different themes becoming more popular.
You can also put up slogans that are associated with them. Make sure that the banners and slogans are big enough to be legible even from a distance.
You can go with a complete romantic theme. Keep the color code as white, red, and golden. Have heart shaped balloons everywhere. You can put up photographs of the couple all over the place. Another theme that you can go for is the tropical theme. You can have bamboo and trees in the hall. Take it a step further by serving the drinks in coconut shells.
You can have a haute couture party. Play lively music. Keep a section for dancing, and set round tables with chairs around the dance area. You can also arrange a ramp for the couple to make a dramatic entrance. Keep the venue slightly dark.
If the couple is into sports, then you can have a sport theme. If they like basketball, you can have an NBA theme. Have hoops all over the place, and let there be ample basketballs for people to shoot hoops with. You can have a contest for shooting the most hoops in 5 minutes, or something along those lines.