Hello! Will be very glad to help you out!
1. Be sure to train thrills: in order to ensure that your hammers and pulls are even sounding, practice 2-note-chromatic thrills all over the fretboard. For this type of exercise you will be using two-finger combinations, for instance: fingers 1-2; 2-3; 3-4; 1-3; 2-4; 1-4 and so on. The objective will be to make each pulse of the subdivision even in volume. Do it as slow as you can, do not be afraid of going very slow.
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2. Emphasize on fingers 3 and 4: Those guys tend to be lazy and require more training in order to develop good hammers and pulls, so always give some extra workout to those fingers. This exercises can be alternated between a 1-3-4 fingering or a 1-2-4 (strech) fingering.
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3. Pull-offs on Descending legato: You may already know, but the rule of thumb is that whenever you ascend you pick the first note of the sequence going up and, when going down, you always hammer the first note from nowhere and then just keep pulling throughout the lick or whatever you are doing. Try and emphasize on that HAMMER ON FROM NOWHERE. Repeat many times the hammer on that you gotta do when switching strings until you can get a clean cross-string hammer on from nowhere.
3.1 Once you get the items above straight, go on and practice licks or etudes with musical meaning so that legato can be more part of your expresive playing rather than a mechanical notion.
4. General Noise Control: Background noise control is essential when using high gain. Your most important tools will be your index finger on the fretting hand and your palm on the picking hand. This is a little bit tricky to explain in writting, but I'll give it my best shot. Your index finger on the fretting hand has two functions when it comes down to noise control: its "belly" has to mute all the strings as if you where placing a barre chord, but without tension, whereas its "tip" has to be in contact with the string above the note that is fretted in order to keep said string muted. As you move up to the higher strings, the less you will be able to damp with your index finger. That's where the palm of your picking hand comes in, you have to, progressively, mute the strings that are let go by the index finger with your palm.
Let me know if this helps you out. It may be kind of tricky to figure out from text, I should probably shoot a video about this. If any just let me know and I'll try my best to help you.