If you look at the variables comprising expectancy, namely ave loss, ave gain and win %, you will find that they always tend be in balance. With that I mean that should you have a system and suddenly adjust your profit target downwards, your win % will increase but your ave win goes down leaving you with roughly the same expectancy. Therefore any such adjustments have the potential to leave you in a roughly similar spot as before, although the road you travel to get there will be different.
The best way I found to solve this problem was to really let my winners run, i.e. get trades on for a couple of weeks (I trade stocks) or longer. However, this introduces its own set of problems. I guess there are no perfect methods, merely methods where we can live with the downsides.
I found it best not to fiddle with stops. In cases where I tighten them, I get stopped out and then the trades turns profitable. In case where I loosen them I usually take a larger loss. Once you've landed some winners, it does get easier to just let the trade do its thing. It also helps if you can trade smaller to protect the account during losing streaks, and if you can add size after landing some winners.
There is no trend...
To clarify the above - just because a market has trended does not mean it will continue to do so. You can lose quite a bit of money even trading in the direction of the trend if your timing is off. However, if you have a trade that works, then at least you know that you are right on the direction at that point, and it might be worth considering holding the trade for longer. To offset the uncertainty, you could try scaling out of winning trades at certain pre-determined points - just be aware that you still want to have sufficient size in the trade so that a big winner will matter.