Pray for them!
Let him be the intention for your next rosary or another form of prayer. At your next holy hour consider spending a few moments praying for your Priest before the blessed sacrament.
Establish a relationship with him beyond a confessor and priest
You don't have to become best buddies but try to establish a relationship beyond him the bounds of Priesthood. Even if it is just saying hello after masses. Being comfortable with your priest will help make confession less awkward and more comforting.
Invite him over
Our favorite thing to do with our local Priest is to invite him over for dinner. Here lately that also included a seminarian or two. The laughs and memories we have made over these dinners are priceless and will treasure forever.
Always be supportive
Verbally let your Priest know you support him through whatever he or the parish maybe going through.
Offer to volunteer to help with the little things
This will help take the minuscule tasks off of his plate, in turn allowing him to focus on new things to grow the parish and to get creative in providing pastoral care.
Encourage them to take time off
Encourage your priest to take a vacation, not just take time off for a conference or mandatory training. Even if it's just to get away for a day. It will encourage and refresh them in turn preventing burn out.
That was a beautiful mass, thank you.
Your homily was amazing! I enjoyed learning about ______.
Money to offset a certain expenses.
A medal or prayer card that made you think of him.
One of your favorite religious books.
A card on his birthday or ordination anniversary.
Go to mass, confession, and pursue being holy
What priest wouldn't love to see one of his parishioners pursuing being holy? This is the ultimate compliment and act of encouragement. If you're only going to mass on Sunday mornings, try going to a week day mass. It's a great pick me up during the week.