Hmm... I have a few thoughts. Thought one always live your life as a witness. ala, brother Andrew "Preach Always and if necessary use words. I have seen time and time again people witness all the time with there mouth but live a non-Christlike life and drive more away then they bring. Do not get me wrong though, I love to witness and I love to lead people to my Jesus. So if you are always living a Christlike walk then you are always witnessing, as it were, alone or with others.
Now if you plan on witnessing alone I would highly suggest two things. One which most have mentioned, stay in public areas not just for safety though, but so that you can avoid all appearance of evil. For instance. If you go door to door witnessing and you are invited into to the home of a single women. If you go in alone, you have set your self up to be accused of many things, even if you don't do them. I heard that Billy Gram does not even ride an elevator alone with any women other then his wife, so that there is no possibility of him being accused of any wrong doing.
It is also advantageous to bring along someone who can either help with the conversation, or sit back and pray quietly.
So witness everywhere you go, at all times. If you plan on going out as something you are doing, go to public places or take a friend. One last thought, always be prepared to make disciples. Just like a good baseball swing proper follow through is imperative. I have led many many people in the sinners prayer, but only a handful did I feel really met with the Lord. When you lead them to Him, be prepared with a Bible, and ore some sort of follow up plan to get them into the body. If they pray a prayer and have no more contact with anyone to disciple them it becomes harder for them to fully pursue the Lord. However we are not the Savior, and Jesus is bigger then all of that, of course, just friendly advise. I was caught off guard with a divine appointment in a wal-mart parking lot, and led a y young man to the Lord. He was from another town and I directed him to the only church I knew in the area and all I can do is pray. I did not have any Bible or any other follow up literature to give him. But I trust in the Him to take care of it! Anyway I hope this helps.