I got Musical Composition from Brindle (1986) and below are some of the key tips on composition he gives:

• To improvise well, the performer needs to know about melodic phrase construction, formal principles, stylistic dictates and idiomatic limitations.
• Get to know many scores and find out how the music is made and why and reapply techniques (rhythm cells, melodic lines, harmony, counterpoint, etc)
• Start to compose with scribbling of any ideas (chords, melodies, rhythmic patterns, etc) on the blank page to make the task less daunting.
• Vision first: If we can’t start it is generally because we don’t have a vision of what we want to create.
• Take concrete decision to make a start: kind of music, instrumets, movements, forms, establishing theme, etc.
• Consistency: the basic character of the music must be uniform throughout
• Save Time with Theory: knowing the exact building blocks of our trade is a great saver (what scale are we working with, what key or mode, what chords fall within the scale? etc).
• Too much detail when we begin a piece can be a handicap (start with a memorable and simple theme).
• Melodic music: Write the melody first (at least the main parts) and then build the harmony around. Don’t work on both at the same time since too early concentration in harmony can obstruct the melodic flow. Same is true for music that begins in other ways (chord progression, texture, etc)
• The beginning is the most important moment of all. Come back to it regularly and improve it.
• Before you play, think, imagine in order to avoid falling into certain patterns we are accustomed with.  After months, you won’t need to play what you are writing to know how it will sound.
• Good music can be written which does not abide formal principles but in general, which abandons formal principles to the point of formlessness is doomed to failure.
• Good form ensures that the emotive message is convincing, unified and complete.
• Statement and Change: Form is about keeping unity while introducing change to keep the interest (remember example of acropolis in athems: Parthenon – Erechtheion). This is the basis of all successful conventional music forms: The alternation of something familiar and something new (Statement and Change); for example ABACADA or ABACABA (A = recurring material anb B,C,D, periods of change)
• Using repetition wisely will save you work and produce coherent work. If there is no repetition, the music is almost certain to escape the memory.
• Rhythm motif: Beethoven used to repeat a lot simple rhythm cells combine with changes in pitch, new counterpoints, etc.
• If the emotive flow is to even, interest will lapse, if changes of emotion are too abrupt, the unity will be destroyed. Allowed change depend on whether the material is placid (less change needed) or emotional.
• Techniques to keep form while improvising (improvisation on a ground bass, on a theme, or using a skeleton harmonic). Think for a moment before beginning, searching for some idea, however brief, then apply repetition and change!
• Form and mathematics are generally interlinked although sometimes we are not aware of the mathematical progressions that make music structured to the ear of the listener.