i find that a good thing is to be aware of what defenses will counter your approach options. for instance, approaching with fair or a magnet for shield pressure will easily take care of a shield happy opponent. on the other hand, maybe the person you are facing is roy, marth, ike, GW, or someone else that can counter a lot of your good shield pressure approaches by purely outranging and out prioritizing you. in this case, you might want to resort to DD punishes, or PKF. maybe your opponent is ness, that has no trouble jumping over PKF and throwing out pk fires, and mixing in fair to approach from above. maybe here, you need something quick with some range to get under him, such as a dash attack or a DACUS, or maybe you use some magnet tricks to help counter the pk fire, and then you can DJC out of it if he tries to challenge you.
I find that its really all a game of rock paper scissors. you have to be aware of you options in neutral in relation to your opponents options that can counter you. That might help you to decide which options are best to mix in as approaches. it probably wouldnt be useful to try and mix your opponent up by going in for shield pressure with magnet/fair, and then the next time switching to a grab. your opponent at that point would be conditioned against shielding (since you just showed them a shield pressure approach). they would likely remember that and try DDing for your grab approach, and punish you. instead, if you did a PKF, their DD would not really have served any purpose but to probably force them to the edge, put them in their shield, or put them in the air. all favorable things for you. Obviously the mindgames go deeper than that, but thats the general approach.
TLDR; rock paper scissors shoot. know which of your options fall into the rock category, which fall under paper, and the ones classified as scissors (in relation to the specific character you are fighting of course).