The degree to which the planets work with one another either natally, or in transit, is based on a few simple rules.

The first and most important rule is gender(also masculne and feminine or odd and even). In the ideal chart where every planet is in its own sign, the planets maintain a perfect separation or synchronicity with each other. That is, the fire, earth, air, and water planets all work the way they should with each other.

The second rule is element. Planets in the same element(fire, eath, air, or water) will blend very closely with one another, whether they are compatible in nature or not. Sun and Jupiter in the same element, for example, will blend very easily because both are of kindred natures. Sun in Saturn in the same element, on the other hand, will blend as well but in difficult fashion as both have very different or antagonistic natures.

Finally, there are planets that are in the same sign. These will blend the strongest, for better or worse, and must be considered as a unit when interpreting a chart, relationship, or transit.

I have noticed, for example, that planets in the same gender as the Sun, will color it most strongly, and take on its characteristics. Even the sextile does this. Someone with the Sun sextile(by sign) to Neptune for example, will be shyer or more sensitive than someone whose Sun is not linked to Neptune(by gender). Sun in Leo, for example, is normally proud, showy, and confident, but if he or she has Neptune in Sagittarius, they will be more quiet, sensitive, and withdrawn. They will likely find an ideal outlet for expression in a Neptunian field such as film, TV, or the arts. A trine or conjunction(by sign) between the Sun and Neptune will of course emphasize this even stronger.

Another example is someone with the Sun and Moon in conjunction, opposition, trine, or sextile(by sign), but with no planets in Cancer. At first, one might think that this person has no strong domestic leanings, but the Sun/Moon link dispells this. I know many people with Sun trine Moon(by sign) for example who are homebodys and collectors and careworkers, even without planets in Cancer. In such a case, the Sun is "domesticated" and the Moon is in turn emboldened or strengthened.

A final example is the Sun/Saturn link. I often see people who appear to be older, wiser, leaner, meaner, and more responsible types with no Capricorn planets, but upon examining their charts, I see Saturn conjunct, trine or sextile their Sun(by sign), and then it all makes sense. Yes, aspects can be this simple and powerful.

I use the Sun in these examples because it is a highly central or personal influence, but other planet combinations can be just as affected, such as Moon/Mars or Mercury/Venus, etc. In fact it is good to make a chart or diagram with all of your planets(including Ascendant and Midheaven) divided into two groups according to gender placement, and then sub-divided by element and sign. This will show you which planets or parts of yourself are closely working with which, and which are maintaining a safe distance.