To those who suggest to use a tuner: this is useless. There is no point trying to play "more in-tune" than what your ear can detect (please note that your audience will also not "listen" to you by looking at a tuner, but with their ears).
Does this mean we should be satisfied if our ears do not detect any intonation issue? Of course not. We should work hard on sharpening and improving our pitch and interval detection skills. It is a lifelong battle.
The way to work on this is to use a keyboard and practice hearing - and singing - intervals. You should particularly work on the most consonant intervals (and their inversions): octaves, 5ths, then majors and minor 3rds, then major and minor 2nds, and finally tritons (all other intervals are inversions of those).
On your instrument, start by practicing playing octaves and 5ths. There are 2 reasons for this:
1. they are the most consonant intervals and therefore those for which our pitch detection is the sharpest.
2. they can be played in position (without shifting) with a very relaxed hand shape; (I personnaly prefer playing them with 1 and 4, à la Simandl, but if you go for 1-2-3-4 technique, play them with 1 and 3 consistently). This will help you establishing a rigorous hade shape and enable the so-called "muscle memory" (of course muscles have no memory, and memory takes place entirely in our brain, but this is a convenient way of talking about proprioception).
Practice with closed eyes, focussing your attention on what your ears tell you. Sing the interval before playing it (you cannot play in tune what you cannot ear and sing in tune).
Then practice other intervals you can play in-position.
Then practice shifting, starting with small shifts.
Do all this with closed eyes (at least for part of your practice). Of course, when performing music, you will open your eyes and also watch your fingerboard (to use all visual cues available there), as well as all other things you need to watch (sheet music, band mates, chicks in the audience, etc.), but closing your eyes while practicing will help sharpening your ears and train your muscle memory.