"help him do" or "help him to do"?

I’ve been always confused about which to use, ‘help someone to do’, or ‘help someone do.’

I kind of figured they both can be correct, but if i SHOULD choose one of them, for example when I’m writing formal documents, can you please let me know which one it is?

They are both correct… I don’t think one is any better than the other. I’d probably have used the “to” variety in the above example, but I’m not sure why…

I’m not sure why I’d use one over the other either but I’ll give some sentences to put it into context:

We need to help the baby drink his milk. (without to)

She helped him do his homework (w/o)

He helped her fix her sink, after it broke. (w/o)

Dancing helped her to lose weight (I could go either or on this one)

Weightlifting can really help a sportsman to create muscle. (could go either way on this one too)

I think of any where ‘help to do’ is the only way it sounds right.

Thanks Jamie and blindside70 for your help!!!