Get the scale right
Most players rely heavily on their ears to get them through - and so they should but you are always going to play some BIG bum notes if you don't have an intellectual understanding of what key you are in and the implications for the bank of allowable notes. So take some time to work out the chords - ask the keyboard player or guitarist, do the transpositions, work out the scales at home and work out what works and what doesn't. It's called 'preparation' people! It's about being 'professional'.
Use your ears
I reckon you should be improvising based half on using your ears to guide you where to go and half on your knowledge of the scale and key you're in. To train your ears to be better ears the best thing is to listen to the style you are trying to play. Nothing beats really immersing yourself in the music of the best practitioners of your chosen style.
Modal Scales
If you have grown up with a conventional music education - grade exams and all that - then you will have probably learnt to be pretty scared on modal scales and feel you're not entirely sure what they are. Well, let's change that straight away! Modal scales are really very easy to understand though they have some fancy ole names! If you play a white note scale (no sharps or flats) starting on C then you are playing C major (sure - der!). But you are also playing the Ionian mode. Then, if you play a white note scale from D to D then you are playing the dorian mode - E to E the phrygrian mode (beginning to get how simple this is?); Lydian - F to F; Mixolydian - G to G; Aeolian - A to A and finally, the devil's scale: Locrian - B to B. I reckon it is well worth spending a few minutes learning the fancy names because then you start to sound like a real muso and that gives you confidence and clout.
That's the 'C major' way of looking at it; D to D white notes is either called the dorian mode of C, or D dorian. Get it? E to E with an F# and a C# (i.e in the key of D major) would either be called the dorian mode of D or E dorian.
Modal scales are much more common, especially in Jazz, than you might think. Learn them, make them your friend.
Rock 'n' Roll
The big one to get your head around (and impress your friends) is the mixolydian scale - e.g. G to G white notes. This is the rock and roll scale. It is the same as an ordinary major scale but with a flattened 7th note.
Shape
As with all my blogs, I feel it is important to get in some basic theoretical grounding before moving on to the practical side of things. So it is with improvising. Here are some other tips.
It is sometimes great to create an overall shape with your solo. For instance, start gently and subtly, with some long, swelling notes and simple, direct phrases but then gradually getting faster, busier and louder before bowing out with a flourish.
If you are following another soloist it is a good idea to listen to what they have been playing and then start your solo with a dramatic contrast. So if someone before you has been showing off their Bebop credentials and flying around like a blue bottle then come in with a long, high roaring blast - YEAH! That's enough widdling!
Never be afraid to play tunes with your solos; it doesn't have to be about showing off (well not entirely) and some of the best solos simply create new tunes over the existing chord sequence.
Be rhythmic
One of the biggest stumbling blocks for the intermediate improviser is that they fail to play entirely in time. Their focus is often on getting the notes right and navigating the chord sequence and they forget about the groove. It is very difficult playing with someone doing this and school bands can actually fall apart because the soloist is playing fast and loose with the pulse. Nail your solo to the beat, feel the groove in your pants and stick with it like glue. Crucial stuff.
Licks
Eddie Harvey calls them your 'dynamic library', I call them licks. Licks are short musical phrases that you pick up along the way and help you to build a solo. You've heard a solo and copied it on your instrument and then you start to reproduce that whenever you're playing in that style - it's a lick! Very useful things, licks, and worth working on and collecting.
That's enough for now. Keep practising...
