I need to recaulk around my bath tub. In some of the areas the gap is
quite large (1/2-5/8" gap) and this makes it difficult. Does anyone
have any helpful suggestions?

I'd be wondering why the gap happened? If it's an old house and this is a
slow prgression (IE: Now it's that big but very slow developing over
several years) you may have normal settling. Is it a downstairs bathroom on
slab construction or something like that? If it's upstairs or over a
basement, I'd say try to access (if possible) where the base of the tub s to
be sure you dont have water damage. Such things are normally pretty easy if
it's over a basement.

Most likely it's just settling but a simple check to be sure is warrented if
you can access it.

Also are there any tricks in general to make the project go nice and
smoothe with good results?

Sure. If the gap isnt deep, just use a good silicone and fill it then there
are several tools to smoothe it. I see some say finger (works with a little
experience to get it even). I have a few spots that need caulking about our
house and we found the back of a small spoon wored well. You recall the old
McDonalds itty bitty plastic spoons? We have a few things shaped like that.
You may want a tad bigger. Then, use an old tshirt to wipe up any overage
before it dries.

If the gap is deep, I see another mentions a filling rope sort of thing to
bring the level up.

Where is the gap? At the base? Or along the top and into the wall? A
vertical oriented gap is a little different from a horizontal one but not
all that much so.
xxcarol