If it's in a pub I'd stick to the natural wooden tables they presumably have rather than white table cloths, and don't go overboard on centrepieces. I actually think pub weddings can be some of the loveliest as a lot of old pubs have so much character!
-If there's a fireplace, make sure a fire is lit in the evening/all day if it's in winter or cold
-Small flowers or plants in silver bud vases dotted about the tables and surfaces, though not too many. I'd go for heather or something lovely and simple, though I'm also totally obsessed with the look dried roses have, which you could cut short for bud vases, and are miles cheaper than standard roses.
-Candles candles candles! Don't use the main pub light, they're often so unflattering. Instead, put as many candles out as you can, perhaps some oil lamps too because they give off more light. One thing I've been doing is buying old pastry moulds and using them as tea light holders because it's much cheaper than buying actual candle holders, and the look is striking! I have about 50 if you want to borrow some
-Fairy lights. My favourites are the ones that are a string of light bulbs, but any will look amazing. Drape them across the ceiling or just hang them across a wall. They always make things look magical.
-If it's a true country pub, white bunting can look lovely.
Do you have a theme or feel that you want to emulate?