Following a regular schedule of maintenance (see the Owner's Manual and the Factory Service Manual) will help you find signs of impending problems which you can then fix before total failure. Don't ignore new sounds from the engine - check them out to see if a problem is developing. Brakes are a critical part of your bike - check them regularly. You can prevent many roadside failures by simply examining the bike regularly while still at home.

Over the course of many years, the following problems have been shown as 'common' for various reasons.

Clutch Spring Plate Failure
The Spring Plate has caused untold headaches for many riders. When the rivets fail they get caught in between the friction & steel plates, causing the clutch to malfunction, damaging the plates & the clutch basket. This problem is best addressed before it happens. The XLForum did an ad hoc poll of those who have or have not had failures of their Spring Plate. The poll showed the rubbermount models (2004-later) failing at double the rate of the frame mount models (1986-2003). The reason for the failures is unknown (??? due to a change in design and/or material, from rider increased horsepower or from the way the rider uses the clutch. ???).
Described in the Sportsterpedia HERE

1998-2003 Nosecone Ignition Failure
The 1998-2003 Non-Sport model used an integrated ICM on the Sensor Plate. These OEM modules had a high failure rate. The MoCo stopped providing replacement units as soon as they were allowed. The only choice for replacement is aftermarket versions (such as the Ultima version).
Ultima Described in the Sportsterpedia HERE