Plenty of fat helps in keeping one regular. In addition, eating plenty of probiotic foods can certainly help. You could supplement with pills, but fermented food such as veggies like kimchi, sauerkraut, ect. I have read have more probiotics than the pills do and are FAR cheaper.
Also, caffeine binds to bile and helps create "bulk" in that way, so it can help keep you regular. If you're using the caffeine as a crutch, you have issues that need to be looked into. However, the occasional use of a few cups of tea or coffee when you feel particularly constipated can be a great relief.