how to make baby/todler clothes last

My baby boys grows so fast. He grows out of all pants and rompers, almost of his shirts becomes short. He is just 4 months but must wear size 6 months.
So mama must do this:

MAKE his clothes LAST

Before doing that, I was sure that crotch still fits.

1. add cuffs to pants/sleeves which are too short (not short sleeves) or romper suit
My express way: use often old baby shirt sleeves for making cuffs. Save so much time for hemming and  sewing. But you can do it with a folded-twice-fabric-tube too.

Pants before:

after:

For the romper suit I cut off footies and added cuffs instead.
Footies to footless.
The left romper suit is size 6m and the left ones is size 4m

before:

after:

Sometimes I didn´t add cuffs, but saw a tube of knit fabric to make it longer (see pic below).

Here I used the old cuffs of the blue pants above because the color matchs. Nothing wasted. 

(I didn´t do iron to show you that used to be cuffs)

This ones I make cuffs from knit fabric tube (folded twice)

2. Turn romper suit (playsuit) into pants
my express way: use bottom of a baby shirt for waistband.

This way are great for romper suits which are too short (or crotch fits no more)

before:

(I forgot to take a picture before. So I put the cut-off top on my baby to show you that the suit was too small for him. He couldn´t wear it on.)

after:

If no baby shirt on hand, you can certainly sew knit fabric to make a loop/tube which fits your baby belly. Here I made a high waist band to keep his belly warm in winter.
Oh, a BABY BELLY BAND. I must do it.
And if you want to lengthen short sleeves, there are already many detailed tutes, like this
http://samstermommy.blogspot.com/2009/04/waffle-tee-onesie.html