For physical subs I have no advice. But here’s some tips for any other type of sub, mainly for behavior modification. You could probably work them into physical type subs if you wanted.
Use present tense, like you said.
Brief is good but not as necessary as many would have you think. Your subconscious is plenty intelligent.
Use two gender voices. Have your gender speak the affirmations in First Person as in “I am”. Have the opposite gender to you speak the affirmations in second person such as “You are”.
If you are doing this just for yourself and not selling them, borrow some technique from Eldon Taylor of Innertalk. His techniques work wonderfully. Have the male and female voices speak in a “permissive” language into your Right Ear, going to your Left Brain. Permissive means you start every statement with “It is OK”. Do the opposite with your left ear or right brain, state everything as a direct command.
Left Ear “I always get my homework done on time” “You always get your homework done on time”
Right Ear “It is OK for me to always get my homework done on time” “It is OK for you to…”
Then reverse the track for your left ear. So your right brain hears everything spoken backwards.
----------End of Eldon tips.-------------
Next. Use the principle of making new connections. And erasing old connections. You are changing your beliefs. All of your beliefs are built out of connections.
Examples.
New Connection: “I feel that having a clean house is worthwhile.” (clean house)+(feels worthwhile)
Disconnecting: If you have a habit of choosing the wrong friends, just for example. “I never view someone who is sadistic as being my friend” (friend) - (sadistic) You are breaking an old belief.
Don’t worry about using negatives. The subconscious understands words like “No” “Not” and “Never”
Lastly, aim at your real goals to get the most powerful results.
Don’t just say “Being financially independent is worthwhile”
Go more for something like “Being financially independent allows me to feel the true worth I already have”.
Find your real motivations and connect them to the habits or outcomes you want. You don’t just want to be rich. You want to be rich for a reason. You want to be rich so that you can feel a very specific feeling. Identify that and connect it.