Think in groups of bars

Even if it may seem outdated and soooooo "classical" to you: There is a reason why most composers think in groups of 2, 4 or 8 bars when writing lines. A melodic arch consisting of an even number of measures, with two parts of the same length is pleasing to the listener. It allows you to let phrases flow. Because the listener hears a familiar form, s/he can concentrate on listening to the things that make your particular music special. Instead of focusing on the alienness of an unfamiliar structure, there is room for a deeper emotional connection. Of course there are situations where you will want to break that "rule", but in general try to stick to it.

Major is not the only mode

This one is for my music students, who constantly surprise me by only using Major chords :)

It is quite funny to see how lots of people seem to forget there are other modes, too. Use them! Take minor, for example: Even if your piece is not set in minor, you can and should nevertheless use minor chords in it. The minor parallel of a chord can easily be used as a replacement. If you want to use F Major, but do not want to stress this resolution to the tonic so much, use the parallel minor chord of d minor instead! You can also use minor chords for starting modulations into other keys. Also, keep in mind that minor chords sound different from Major chords, but not necessarily "sad". This is what they told you in elementary school, because it simplifies things nicely. Trust your ear!

Not to mention the existence of church modes... these are a totally different world and incredibly useful and will be the topic of a later tip.

Give them their first beat!

One thing that annoys musicians tremendously is composers writing a nice, soaring line for them, but not allowing them to play a last, short note on the first beat of the next measure when their line has ended. Somehow we are internally wired to expect our parts to finish on a first beat. The most prominent example of this is the end of most pieces: After a (lengthy) final cadence they tend to end with a big "bang" on a first beat. Try to compose this way in the whole piece and your players will have much more fun playing your stuff!