1. Winds + Brass
A very powerful combination, it can be divided in 2 section:
1.1 Winds + Brass in Unison
This is a very common combination, which is used in many classical and contemporary pieces. Since Brass instruments are much more powerful and aggressive (because of that "cutting" sound they produce), the Brass tone will always dominate in this combination. But the Winds will make it much more soft and sweet, it acts like a soft pillow underneath the sharp layer of Brass. So, if you want the melody to be kept up by Brass, but also to decrease the tension or transition to a smoother and calmer passage, this combination is a very powerful tool to keep under your belt.
The trumpet is most frequently used in this kind of combination, since it fits will with many winds, such as:
Trumpet + Clarinet
Trumpet + Oboe
Trumpet + Flute, or even all those four together.
Some other frequently-used combination are :
Horn + Clarinet
Horn + Bassoon
Trombone + Bassoon
Tuba + Bassoon
1.2 Winds + Brass in Octaves
It may be a good idea to double a Horn with a Trumpet in octaves, but often you may not be satisfied with the result, because of the low efficiency of the Brass instruments in the highest register. In this situation Wind are your saviours. Try doubling the Horn in Octaves with clarinets or flutes. This won't change the colour much, but will add a sweet harmony.
Also doubling the Trumpet itself with Brass in octaves is a good idea - the sound will be much more richer, since Trumpet isn't capable of producing such dynamic sound.
But be careful, not every Brass instrument should be doubled by Wind, for example, Trombone will sound very muddy if doubled in octaves by Flutes or Oboes, in this situation it's much more convenient to use a Trumpet, it will produce a nice resonating synergetic sound.
2. Strings + Winds
Basically, grouping winds with strings is a great idea in almost every situation, since they resonate very well with each other and can always be used to slightly or drastically change timbre/color without changing the harmony or melody.
The basic methods of grouping Winds with Strings are:
2.1 Strings + Winds in Unison
This can be used to either change the tone/timbre/colour of the current sound, add some power by increasing the resonance between these two groups and, as in the Brass + Winds situation, make the Winds sound softer and not so aggressive.
Best combination usually are:
Violin + Flute/Oboe
Cello + Bassoon
Double Bass + Bass Clarinet/Contrabassoon
2.2 Strings + Winds in Octaves
This combination doesn't have to desribed as the previous ones, since it follows all the "rules" described above. But keep in mind that this kind of grouping is very massive and powerful and creates that "epic" movie-esque sound.
Listen to Mozart's Marriage of Figaro for an example of some huge String + Wind groupings.
Stay tuned for the next part, where we will be talking about harmony and grouping for different harmonic effects!