'Practise more' is the essence although it might be worth making sure you're using the time you do play efficiently. You can throw hours and hours at a tune but if you're not doing it in a way that works for you, you may find you don't make much progress for the time spent.
Usually you can pick out individual sections, phrases and bits of a piece that make it hard, it's pretty rare the entire thing is a nightmare. So I have a play through at a tempo good enough to hear the tune and get a feel for the piece. It's important that you keep playing if you screw up a turn or misread a note, don't stop and don't repeat the bar etc. Plow on, but make sure you remember the bits you couldn't do first time. Next go through and pick out the bits you need to work on, I often need to circle the odd note I keep misreading or forget an accidental etc.
So with a particular bit, depending on what you find hard with it - be it purely fingering or a combination of the notes aswell as the timing, I find it very helpful to take a step back from the stand and lower the instrument. Look at the phrase properly, think about the notes and what key they're in (I often get tripped up sight-reading triad arpeggio triplets, so I find it easier to think of it as a chord rather than individual notes).
Next, sing it. I try to sing the phrase while tapping my hand to the beat. It's important you get this bit right if you do it, because it should help form in your head what you're expecting to hear when you play, so you can be thinking ahead of the notes rather than just 'reacting' to the notes you see on the page. So as with playing, start slowly if it's a tricky rhythm.
Only once I'm absolutely sure I know how the phrase I'm trying to play is meant to sound do I try to play it on the instrument, and again make sure you do the full thing at a speed you can manage. Go as slowly as you have to, and only when you can play it 100% at a given speed should you try playing it faster.
If it's a new tune entirely I often start by singing the tune even if it's not difficult, just so I get it in my head. Over time you'll find the generic base speed at which you can attempt something to begin with will increase and the number of little bits that trip you up will be less, as instead of seeing a bunch of hemi-demi-semi quavers and thinking 'PANIC', you'll recognise it as an Am7 run or a chromatic etc. and you'll find your fingers can just about do it first go, from all those other times you slowed something right down and worked through a singular phrase perfectly.
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That's the way I've always gone about learning the hard passages, the main thing that really helps is being able to sing it, it sort of changes the mental process of reading the music into preemptively thinking ahead what notes you're going to play next.
It might not work for you, but you should give it a go next time you find you just can't get your head (or fingers, rather
) around a phrase. Sometimes a fresh approach makes all the difference.
