1997 7.3 Getting harder to start

[DougC]

1997 F-250, 7.3 liter stock, 78,0000 miles:

I know this is probably pretty basic, but it's my first diesel. Ambient temperatures around here are getting down to 30 F or so at night and around 45 F during the day. The truck is getting hard to start. Before the weather cooled down, it would start right up as soon as I turned the key after waiting for the "wait to start" light to go out. Now it takes three or four tries. What's the best way to diagnose so I don't go wasting money on unnecessary parts? Any tips would be greatly appreciated.