Put flagging tape on the fence- it is cheap and makes it visible, although it doesn't look the best. You can get it in pretty colors though! I have... -cough cough-... 70+ rolls here from my dad (ex-forester). Blue Flagging Tape S-6089BLU - Uline
High tensile wire tends to blend in into the surroundings a little bit. You can also put up a strand of electric rope if you would rather.