Building Shoulder Strength

Strength increases as you do more, but adding in different kinds of exercises will help, both in speeding things up, and by strengthening different sets of muscles. It's a circle... the more muscles you're using, the faster you get stronger

Pullups
Pushups
Climb ladders
Climb ropes
Arm circles (small and large)
Tray carrying (think restaurants)
Heavy backpacks (fill 2 liter bottles with water)
Backwards pushups (off the edge of a couch)
Lizard walking (think kids goofing off, or no wheelchair)
Low crawling (army guys)
Crawling (yep, just like a baby)
Handstands (against a door if your balance isn't good, yet)
Handstand pushups
Dance
Sex (quite a few positions)
Bucket carrying
Kid carrying
Tree climbing
Swimming
Massage giving
etc.

The more you do that stuff the more your shoulders will adjust, plank is a killer it took me awhile too.

You could try downward facing dog pushups.

But down dog without pushups will help too.

here's a simple exercise: place the palm of your hands on the wall & do push-outs! Everytime you pass a bare wall....do a set. You'll be amazed at how much it helps! :)

Congrats on your exercise regime!

For shoulders, I use free weights and do overhead presses. That really helped, almost as much as my toddler requiring frequent picking up!