I did this ride last year with Go Cycling Maui with three other riders, I would bring shorts, jersey, socks, shoes and pedals. They provide support including food, water and nutritional supplements. They stopped at various stops for food. The road is pretty well marked and becomes the only road to the summit. The bikes they use are Lightspeeds and get you a bike your size with a quick fit. They only do this ride once a week from what I saw, so you would need to contact them as to which day they are going. GMC does other rides during the week with support and definetly check them out. They stop the support at the park entrance.

I trained by doing a lot of riding and many loops on a local 6% grade hill (Haleakala is avg 6%), its hard but its fun and at 6% not huge steep although there are steeper sections. There is really one hill up on the way back, long enough to wish for more down hill. Most of the down hill people start at the entrance of the park and I wouldnt recommend it to anyone.

Below is a cut paste I have answered to people regarding riding on Maui:

<In Kehei you have a rental at South Maui cyclery http://www.southmauibicycles.com/ and they have some good bikes, bring your own shoes and gloves ect.., they provide seat bag, helmet. There are several other places that rent bikes including Lahaina and Haiku. They do supported rides out of Haiku (and rent bikes) at http://www.gocyclingmaui.com/store.html including to the top of Haleakala ($$).

As for the riding on Maui most roads are pretty good there (avoid Hansen road by the sugar mills) and there is a great trail along hyw 311. Major rides are: North mountains (30 to 340 to 330 to 380, 100km I havent done it yet) up country (37 to 31 until you hit the bad part of the road) or shorter south of Kihei take 31 through Makena. They also ride the Hana Hwy but I havent. Out of Kihei there are great shoulders on the Pi'Ilani Hwy and really better than riding the main road in Kihei. Plan your rides around the trade winds which run between the two mountains and wrap around the two mountains, these come in from the North East mainly in the afternoon. Here's a map of the island that includes great bike lanes: http://www.westmauicycles.com/Maui-County-Bicycle-Map.html>

Haleakala is a great ride and very rewarding, I know people that have done it solo but I would recommend Go Maui as they include a great bike and support.