Mars goes retrograde less frequently than the other planets, doing so every two years and two months, on average.  Therefore, these periods tend to take you by surprise, much like encountering a trip wire during a marathon.  Mars rules Aries and anciently ruled Scorpio, so if your birth sign or ascendant is in one of those signs he will have a stronger influence on you.  Mars is all about drive and energy and if afflicted natally or by transit his energy can manifest as aggression.  If he is retrograde in your chart, you may be the target of aggression or tend to be passive aggressive.

When Mars is retrograde his usual energy is confined and directed inward.  It can cause a tremendous amount of frustration, depending on the sign and house involved.  When Mars trines the Sun a few weeks before going retrograde you may encounter a dramatic burst of energy which will pop like an assaulted balloon when he stations.  Apathy and disinterest is likely to follow, which can present some real problems, depending on the importance of your original endeavors.  Nonetheless, the Universe tends to know what it's doing and it's best to listen to what the message is for you.  If nothing else, resisting the planets and their respective energies tends to be the ultimate exercise in futility. 

At the least there will be an energy shift which will be most noticeable in the house Mars is transiting.  While his direct energy drives you forward, when retrograde you may find yourself coasting.  While some momentum may remain, it will be less directed in nature.  Somewhat as a child may take a circuitous path home from school, you may find yourself less focused, more receptive to alternate routes or other distractions.  You may tend to question your original destination and its rationale.  This will be exacerbated if Mercury goes retrograde during the same time, in which case you would do well to use all that internalized energy to closely examine your original plans and intent for significant revision.  If you've been ignoring anything about your current course, consciously or unconsciously, these matters or concerns will begin to leak out, initially as yellow flags and eventually as red ones.  You may encounter roadblocks, self-imposed or otherwise, which force you to reconsider the chosen path.

If you've been suppressing any frustrations or hostilities expect the heat to be turned up under them, forcing them to build up like a metaphoric pressure cooker, hidden and silent, but potentially violent.  Needless to say, recognizing these for what they are and dealing with them via a relief-valve is much advised as compared to the explosion which could result otherwise.  Conversely, you may simply feel as if you have wandered off the highway to a rocky, rutted and remote country road when in reality you have found the ultimate shortcut to where you really wanted to go in the first place.  For example, if you've been suppressing your true desires for any reason, something inside may finally assume a life of its own and assert itself, demanding respect as well as attention.  If you're in a job or relationship you detest, this may be when your battered ego comes to the fore and screams "Pay attention to me!" with all the class and finesse of King Kong on the Empire State Building.

At any rate, when Mars stations direct, any such matters are going to come to your attention.  If this occurs in aspect to a planet in your natal, progressed or other significant chart, particularly a Mars Return, the effect will be much stronger.  The coming months during the shadow period, i.e. while Mars is retracing his steps to the zodiacal degree where he originally went retrograde, will be a time when you get out the backhoe and start removing all those rocks and obstacles such that you can construct a new path, perhaps in an entirely new direction than where you were originally headed.

Much of Mars' influence will manifest in the house he's transiting.  As he makes contact to planets in your chart during the cycle (especially the station, opposition to the Sun and subsequent trines) the respective house(s) of residence and rulership will likewise be affected.  To summarize, the RX station begins the process, the opposition turns up the heat, the station direct allows it to come to the surface, and the subsequent trine brings new direction, which proceeds when Mars exits the shadow period.  The following table traces Mars retrograde cycles for the past several years.  Consider what was going on in your life at that time, which may help you anticipate what the next Mars RX cycle may hold for you.