It is highly likely that you will uncover duplicate files or uncover opportunities for optimization when you spend a few hours cleaning up a project that has been through several iterations. Are you working on a project without version control? You might want to consider adding that to your skill set. At the very least, never remove files, rather archive them.
Playtest soon and often
Playtest your game regularly and gather player feedback: Playtesting is an important part of the development process and helps you identify and fix any issues with your game. Look beyond your usual network for more honest feedback.
Join a community
You can get assistance from other developers. As long as you treat them with respect, you will find that most will be willing to assist you when you need help with your game.
Create a plan
Having a game design document is good practice, but don't worry if it's not 100 pages long. Consider creating a task list, a moodboard, or some notes. Every little bit helps!
Start with the most important and essential features and work your way down. This will help you stay focused and avoid wasting time on unnecessary elements. Create both the beginning and end of the game (or level, world, etc.) so that the player can play from beginning to end, then fill in the gaps.
Keep it simple
Simple game designs are easier to develop and maintain, and can often be more engaging for players. Make sure your game design is scalable so you can add new features and content as needed.
Never stop learning
There are a lot of resources that offer videos, articles, and tips. Keep up with the latest game trends by following news sources, developers, and publishers!
Have a clear vision
Have a clear idea of what you want your game to be and what you want players to experience. This vision should guide all development decisions and help you stay focused on your goal.
Getting stuck is not a problem
Are you having trouble with a bit of code or a part of your game? Take a break and come back to it later. Consider focusing on other elements for a while. After taking a break from it, you might be able to fix that one bug.
Make sure your game runs smoothly and efficiently on different devices. Optimize your code and assets for performance to ensure a good player experience.