The solution is often relative simple - fish a small fly on a thin(ner) leader. Most of the time, I really don't think that patterns matters much, but sometimes, even a small fly is too big.
I've had success with small Red Tag and Black Zulu on size 10-14 hooks, a small mysis-patterns of my own, less than a centimeter in size, and then this gammarus-pattern.
Gammarus are immensely important as trout food, they're active year round and the they start re-producing already in late winter.
So a gammarus pattern is a good bet year round, and when I fish a two-fly leader (which I do more and more), a gammarus is very often one them.
There's a very famous, Danish fly called Kobberbassen, which is nothing more than some copper dubbing on a small(ish) hook, and there's little doubt that this fly is taken for a gammarus. And of course, a lump of dubbing is so much easier than my gammrarus, and probably just as effective, but none the less, I perfer my own version.
This time of year, there are also the rag-worm hatches, which can priduce some magnificent fishing, and a rag-worm and a gammarus on the leader is a perfect pair this coming month.
If you're interested, I did a tying video of the gammarus (in Danish, I'm afraid) for a Danish magazine. Check it out here.
I tie this pattern from #6 to #10 to use in the salt, and I tie almost the exact same fly (different kind of dubbing) for nymphing for trout and grayling, from #8 to #14.
Have a great weekend!