Hi!
I use a long (30cm) aluminium U-bar and wrap the sandpaper aound it (see picture).
For a good finnish it's vital that you have good sanding papper.
Best sanding paper on theis earth is a sanding paper from Portugal called "Indasa red line, Rhino dry plus " )
This sanding paper is soo good and will last longer than any other sanding paper I've tried in 32 years modeling that if you only have used 3M or Norton before you will be amazed...
I have used it for more than 10 years now and mostly use 180 and 240 grade and a fine 500 before putting the final layer of paint on, using Sikkens Autocryl 2 part auto laquer.

For heavy sanding the best sandpaper so far is a cloth-based paper called "Hiolit" I prefere to use grade 80. This paper is so tough that it sand 24 hour epoxy and microballon covered balsa wings with ease without loosing it's bite or brake.