Watch for a lower sense of energy, and perhaps some despondency. Neptune is a slow-moving planet, but it will move off eventually.
A couple of things to watch for. Neptune does not deal with realty. Some astrologers think that Mars-Neptune relates to lying. You may not be clear on what's true or false, real, or unreal. Because this is one of the hard aspects, you may have to try hard to stay grounded and avoid what you know in your heart to be false, both in yourself and in other people.
Neptune also rules drugs and alcohol. If you're feeling low, you may be tempted to self-medicate with them. Not a good idea, because you will probably be extra-sensitive at this time. You may want to spend more time in a fantasy world, such as through a computer game, reading, films, or TV. That in itself is fine, so long as it doesn't turn into escapism.
A good time to maintain a healthy diet and exercise because your immune system can also be lowered. If you live near the ocean of a large body of water, you may find it especially inspiring at this time.
Also, look for the positive transits in your life.