When fly-fishing on the Annan there is no real requirement for huge double handed rods. Rods in the 13ft - 14ft range will cover most eventualities in the lower to middle river, although if yours is 15ft don't worry as it will still be very serviceable. At the top of the river a double hander can sometimes be more of a liability than an asset and a heavy reservoir rod in the order of 10ft is probably more suitable. In general floating lines with a variety of sink tips will cover most of the river but it would be worthwhile carrying a full sinking line as well (though not too fast).
Flies are all about confidence and you should use what you feel comfortable with. Size is the most important matter when considering what pattern to use, with colour running at a close second. In general if the water is warm and lowish a smaller fly will be best and when the water is running a bit higher or colder consider larger flies. If there is a bit of stain in the water a pattern with a bit of brightness about it will probably out fish than a more sombre one. Popular patterns in the area at the moment are the Allies Shrimp (and all of its variants), Stoats Tails and various tubes such as Temple Dogs etc. A local pattern that many of the older anglers swear by (and catch an awful lot of fish on) is the Brown Turkey.
If you have a spinning rod as well bring it along. The Annan can rise very fast and become unfishable with the fly at short notice. Spinning or bait fishing at this time can reward you with fish that otherwise could not have been caught. Also when the water gets very low and clear and the fish are starting to get stale, a well-presented worm may be one of the few ways in which you could get a fish.
During the summer months and early autumn most anglers that spin do best on Tobies and Flying Cs. As the water cools down towards the end of the season the Devon Minnow comes into its own with Black and Red, Black and Gold and the Pink Lady being particular favourites in either sinking or floating formats.
Most anglers on the river favour single-handed rods in the 9ft 6in - 10ft 6in range throwing a 7 line. If you are new to sea trout fishing but have fished for stillwater trout the chances are your rod is about perfect and there is no need to go out and buy a specialist sea trout rod. As with salmon fishing a floating line with a variety of sink tips is probably all you will need.
As for the fly box make sure that you have silver and blue flies such as the Medicine or Teal Blue and Silver and Silver Stoats Tail. On very warm nights consider bringing floating lures as well because these can be very effective. It is also worth including a few dries in the box - Annan sea trout do rise to flies and can be caught on the surface readily.
Lighter rods around 9ft and throwing a 5 line are best suited to most of the brownie and grayling fishing on the river. There is little need for anything other than a full floater.
In the spring flies such as Greenwells and Kites Imperial will catch large brownies feeding on the prodigious hatches of dark olives. Come summer such things as Silver Sedge and Tups can imitate the large hatches of sedge and smaller olives. The river holds a population of very large brownies and whilst these can be caught on the surface in the spring by summer they feed almost entirely on fish (minnows in the main) and can be caught on sculpin patterns. During autumn and winter, when the main quarry is grayling, flies, Czech nymphs grayling bugs and klink hammers should find a place in your box.