You can prepare a bath of dilute EtBr in running buffer, at least 5 mg/L, and soak the gel in it after you finish running it. This bath can be reused many times.
You can add EtBr in the loading dye. Or you can bye GelRed, which is comparatively safer than EtBr. People now a days are abandoning use of EtBr as it is not safe to work with and a bath will cause more risk and contamination in the lab. Read the GelRed protocol and I recommend to use it over EtBr.