Set up a good chord progression, usually in a major key. Chord progressions can also have minor sounds, but are generally not more involved than the simple "rock" or "country" genres accommodate.
When imitating Mark Knopfler, keep your guitar solos and riffs "melodic", in straightforward pentatonic and chromatic scale patterns. Knopfler's riffs are not "jazzy" or esoteric. They have a natural and solid continuance. Knopfler's general melodic style distinguishes him from many other great guitarists.
Use Knopfler signature patterns. One of his "walking" patterns is found prominently in "Brothers in Arms" (Dire Straights title track) and "What it is" on his solo album "Sailing to Philadelphia". This fast combination of hammer-on and walking note combos is pure Knopfler.
Use a "clean" guitar tone. Make sure the voice of your guitar is unencumbered by distorting amplifier effects or heavy action on the strings. Knopfler coaxes an almost plaintive tone out of the electric guitar, and the notes, in most of the recordings, sound very organic.
Look beyond Dire Straits radio classics. "Money for Nothing" is perhaps the best-known Dire Straits song around, but its grinding guitar work lacks the complex melody of other Knopfler riffs. Listen to a variety of Knopfler songs to get an idea of how many different sounds he's tackled over the years.
Listen to Dire Straits "epics". Long recordings like "Telegraph Road" (Love over Gold) or "Tunnel of Love" demonstrate Knopfler's style well. Key signatures repeat throughout the songs and you can get a solid understanding of Knopfler's stock riffs by listening to them a few times.