In my experience, the one thing that has made the biggest difference in my play and the play of people I’ve taught is becoming better at tracking where my key cards and the effect that has on the way you tech. (@EricF is really really good at this, and reading some of his games is extremely helpful in developing the way you think about teching and cycling cards).
I’ve seen a couple of people in this thread advise building tech 2 as early as possible, but I think this can be a huge mistake. You should build tech 2 the turn before your tech 2 units arrive. If you read a lot of the PbF games (even among players who are not novices), it’s extremely common for players to build tech 2, and then not play any tech 2 units the following turn. Obviously, you can’t predict your draw, so this may happen occasionally, but it seems to happen a lot. If you watch what people tech, it’s not unusual to see players building tech 2 when they have 0 probability of drawing a tech 2 unit in their next hand. I guess you could be bluffing, but it seems generally to be a bad idea.
Another thing I’m finding recently is that I’m moving out of a mindset of spending all my gold every turn. I tend to advise new players not to be scared of floating gold - sometimes you don’t have any good plays to make, and it’s better to hold your resources until next turn. I also find that when my opponent floats gold, it’s scary, so it seems like it’s a better strategy than novices usually imagine.