Trade journal
A sheet or notes where you record every trade: entry, exit, stop, size, reason and result. After a month you see which mistakes repeat. Memory fails, records do not.
Example
The journal shows that most losing trades were opened without a planned stop and right after a winning streak. Now you know what to fix.
What not to do
Do not record only the winning trades. And do not leave the fields "why I entered" and "how I felt" empty.
WEEX
FxPro
Vantage