Watch the Waltz Solo online guitar lesson by Jason Loughlin from Essentials: Country Soloing Styles
Knowing how to phrase over 3/4 is important but knowing how to show restraint and play something simple and beautiful over a slow tunes crucial. Establishing a solid melody for you to embellish is a solid approach. Sometimes just playing the vocal melody with a few slight variations can make for the perfect solo.
In this study, I start by playing a low simple melody. Then I get into the mindset of imitating other instruments to make the solo sound more colorful. I'll play a phrase similar to what a mandolinist would do and then lead into it using pedal steel bends. If there was a steel player on the session or gig, I would pick a different direction. You don't want to take over someone else's role. We'll using these concepts over a simple progression in C.