Keep your notice board up to date. Use well produced notices. Keep them regularly changing so it looks as though your building is in constant use.
Then make sure it really is - try to meet daily, for prayer, for Bible study, for coffee, to have a sociable afternoon over a game of Scrabble - or a Christian game - there are plenty on the market, to listen to music together, to sing favourite hymns. It doesn’t matter much what you do (as long as regular prayer and Bible study are paramount). What matters is that people frequently see you going in and out of the church or chapel looking happy. They won’t want to miss out!
If your building is not a place you feel confident to invite outsiders to, think what you can do from home - or is there a village or community hall in better condition?