Definitely trip in a familiar environment where you can control as many things as possible, ie, you can be sure no one is going to unexpectedly show up, you won't have to leave, etc. Good music is one of the best things you can control, music of course will influence your emotions (either positively or negatively, depending on the choice), so definitely have some good playlists or CDs or whatever lined up. For me, being out in nature is the single best setting, I almost never get anxiety or negative thoughts when I'm in nature, and the beautiful complexity of nature provides endless sensory input for you to digest. However it needs to be somewhere safe. For me, my yard is a great setting, but that doesn't work for everyone. You definitely don't want to trip outside in a place where other people could suddenly show up unexpectedly.
Having a benzo on hand for if things get dicey is a good idea, but I almost never do... if you have them that can be good for peace of mind, just knowing it's an option if things go bad, but try to avoid using them because they will destroy some of the entire point of the trip.
And of course, don't trip when there is something specifically bothering you emotionally, ie, you are having troubles with your relationship with someone, or you had a really bad day, or whatever. The exception to this is if you're trying to trip to get some insight on something, but be careful with that, wait until you're at a pretty stable state emotionally (for example, I tripped recently to help me determine how I feel about my impending divorce, but months had gone by first and I felt ready).