Use the right leader
Use a leader and tippet that turns over the fly properly. If the fish are difficult, it's a good idea to make your leader longer and finer. Be prepared to go from a leader of one rod length to one and a half or even two. A quick field solution can be to extend your leader and tippet by tying on a piece of even thinner tippet with a surgeons knot, but the best solution is to select a leader that is built longer.
When water gets murkier, rougher or light fades, you can consider shortening your tippet. Darkness and wind will make casting more difficult, and a shorter, stiffer leader will often be very welcome. The fish will be less shy and presentation might be less important. Remove the tippet, replace it with a thicker piece or use the outmost part of the leader as a tippet. Or better yet: replace the whole leader with a leader that's built shorter and thicker.
And don't forget: smaller flies require thinner tippets and in many cases loop knots to work properly, while larger flies will call for thicker and stiffer tippet material.
More about leaders - and even more