Many people consider multi-instrumentalists to be "Jack of all trades, Master of none". Most of the time they're correct. People either practise far too little, so they can't develop on any instrument properly, or their practise routine just doesn't allow for musical evolution. Hopefully this article can help change that.
First things first, make a proper practise schedule! "Practice" does not mean playing a lick a few times, checking on Facebook, and repeat. If you can, know what you are doing this week. If there are days you cannot practise, that's fine. But try not to have too many days in a row not practising (I personally think 2-3 days in a row is too much, but some would disagree).
I'll show you what and when to practise in the following paragraphs.
I tend to categorise things to improve on into two groups, and the first I'll show you is things that can be done on multiple instruments:
Aural (Aural, not oral ;) ) and transcription training are some of the most important things a musician can improve upon. It improves your improvising and your song-writing massively, and it's quite simple. Say I really like the bass line in Weather Report's "Teen Town" (Okay, who doesn't), I might transcribe this on my bass. But by doing this, I've also improved my aural skills on other instruments too. Another important thing to point out is write down the transcription in notation, not any form of instrument specific-tab. Then you improve your sight-reading and understanding of rhythm at the same time, which can be applied to any instrument. You're also getting to know where the notes are on your instrument too, not just the fret numbers (Sight-reading provides similar benefits). Music theory (like how the modes interact with chords, when to use each interval, etc) is also very beneficial. So to sum it all up, these skills are most important to the multi-instrumentalists, because they benefit all instruments.
The other important group to practise is the instrument specific stuff. I can't possibly list all the instrument specific stuff here for every instrument, sites/lessons focussing on that instrument will show you skills you need. I will say this though; When increasing speed, make sure you're doing everything correct. If you're a guitarist, make sure your alternate picking is strictly up-down-up-down. Make sure the only noise coming from your amp is intentional. Practise whatever you are doing at least 4 times, then move the metronome's bpm up by 1-2, and repeat. And whatever you're doing, relax. Look up Joe Satriani or Marcus Miller on YouTube. Do they look relaxed while playing? Of course they do, because they practise well and are masters of their crafts. Remember, practise makes permanent, if you practise something wrong, you have to work hard to make it right.
Next step: set goals. You need to reasonably think what you would be able to complete within a week, so maybe you transcribe a certain number of bars in a song, or play a set of licks at a set bpm cleanly. If you complete your goals early, good for you, get a cup of coffee and feel proud! If you really wanted, you could set yourself a more challenging goal to complete, e.g.: if I played a lick at 100 bpm 16th notes, maybe I'll try and play it at 110 bpm instead?
Write down what you will practise on the days you can, and start with an hour a day. An hour of proper practise can work wonders for you musically, and as your self-motivation improves, you can practise more and more without "Burning out."
I hope this will help you, and constructive feedback would be appreciated.