1) Make more maps.
2) Think critically and listen carefully to other's feedback. I have a problem with overthinking when I design, so sometimes I just have to accept how things are and move on.
3) Remember that no map is perfect
4) Be creative and don't be afraid of recycling existing ideas or gimmicks for something new (of course, don't fully copy paste an existing map, unless you're making gullywash). Feel free to experiment and create some crazy maps if you like, though
5) Get your scale right and the map is automatically much more fun. If the map is too wide it tends to be a bore, and too small and people complain about it being cramped and getting stuck everywhere. This is kinda hard to accomplish... (at least to me.) TIP: Use stickies and the range of sentries to determine if your map is too wide or small. Compare with existing maps that are proven to be fun
6) If you REALLY wanna do some hardcore mapping, give yourself a deadline and stick to it. I find myself work 500% faster on 72h contests than regularly
having problems poping up in your maps only means that you're not sticking to what everyone knows already works(boring).
But stuff that already works work because they're fun?