#1 trick is to not avoid using your pinky. It's amazing how many "great" players basically only use 3 fingers on their fretting hand. They're effectively diminishing (get it?) their fretting capacity by 25% for no reason whatsoever. No other instruments do this and they never have "pinky issues" because they learn to use them from the start. The same is true when learning to type, or any sort of fine motor skill. The more you avoid learning proper technique from the start, the harder it will be to learn it down the line. Bad habits are hard to break.

This where the whole idea where "proper" fingerings come into play. Just because you could reach a note with your ring finger, doesn't mean you should in the same way that you could lift a boulder with your back, rather than your legs, but that wouldn't be very wise, either. You might get away with it when young, but you'll pay the price in time.

Generally speaking, "every finger has a fret", so if your going to reach from the 3rd to the 6th fret, make yourself use your index and pinky fingers, even if you could stretch. It's no different than using "proper form" when lifting weights. Ultimately, good technique exists to protect you and make you better, but only if you use it.

Lots of acoustic guitar/bass playing will also help to strengthen your left hand. It really forces you to have strong fretting and intonation. Using slightly larger gauge strings than normal can also enhance the challenge.

There are lots of Classical Etudes, "Studies" and exercises you can do, as well. They aren't always very "musical", per se, but think of them like going to the gym, so you can play well on the field later. Check out "24 permutations for the Guitar" if you want just a bone dry, but very rigorous exercise. Another approach is to pick a song you know/like and then forcing yourself to arrange and play it using "proper" fingering, rather than your normal instincts. Some Guitar/Bass exercises will even use indicated fingers for the left hand, but usually only on very simple/basic beginner books. I like all the CAGED patterns and associated fingerings, as well.

Arpeggios are another great way to strengthen your fingers while also training your ear. For example, reaching for the minor third/major third on the low E string using your index and pinky fingers is always a good default pattern to build up the pinky too. Although, there are many other possibilities, as well. Paganini Caprices are great, shreddy challenges that will force you to use proper fingerings. Most non-violin folks learn the 5th and 24th. Something like the Bach Cello Suite No. 1 in G major is another great acoustic practice piece that is also very popular and fun to play.

Ultimately, you just have to do what you know what you're "supposed" to do. Slow down, think about how you're playing what you're playing and make yourself do it the "hard" way. It will only feel "weird"/"weak" for a little while, but before you know it, you'll be up to speed in no time. "Use it, or lose it ;) "

Form = Function