Start with a guest list
Preparing a guest list does not mean deriving an exact number. It just means getting a rough idea for the food arrangements. Anyways the number of guests will increase your estimates and people tend to roam around in groups. But still a rough idea is important. Include your close relatives, friends, neighbors, a few of the neighbors relatives, and kids' friends for the estimates.
Colors & Water
Needless to say, arrange for colors. Make sure you use only gulaal and the natural colors. Other colors might be harmful for the skin. And since other people will also be using the color that you arrange for the party. So keep it safe.
Also arrange for some plastic pichkaris, water pools, water balloons and enough of water supply. Water will be required in abundance. Get adequate water for the party, at the same time ensure you just don't waste it. Holi without water is no fun, So don't do away with it completely. Not keeping a direct supply, like a tap or a pipe will keep it under control.
Food & Bhaang
Music is an essential ingredient for a Holi party. A Holi celebration is not complete without a "Rang Barse", or "Holi ke din" or a "Holi kehle raghubira." Bollywood has some amazing numbers for the occasion. Play them. Or alternatively, you can arrange for a dhol.
Holi gifts are also a very integral part of Holi. Make them feel special by gifting sweet nothings to your close ones for the occasion.