What are your best practices of gaining permission to hunt private land here in Texas? I have left many notes on gates asking to get in contact with landowners but no return emails or phone calls? All the advice would be appreciated!

Offering of work may help, some $$$$ offered may not hurt.

I have done both of those in the letters?

Trying to find a hunting lease near any larger city doesn't come easy.

Check with the chamber of commerce web sites in the areas you're interested in, many list hunting leases.
Check with folks in feed stores, farm & ranch supply stores, maybe put up a flyer in a few of them, same goes for any local gun/outdoor type stores. Regularly pick up smaller towns local newspaper and those local classifieds penny saver/american classifieds/ thrifty nickle type stuff from the areas that interest you.
Put a 'wanted' add in a few that cover the areas you're interested in.

Use the word lease not gain permission to hunt private land.
One sounds like expecting to pay for access, the other sounds like looking for free permission.

Most folks that are inclined to lease have already done so. Other people own small parcels and have no interest in strangers traipsing around the property, shooting. I think your best bet is to search for an available lease. Otherwise, get to personally know people with land and hope for the best.

I personally know lots of people with land but they don't frequently offer up hunting. It's tough, but every now and then something will come through.