An important note about doing this is that you need to learn to know which vowels to leave. Some words will be too easily confused with other words if you omit every vowel, sometimes to the point that you cannot decipher what you wrote at all, but it's also useful for your notes to be almost as easy to read as if they were written out fully. If you're not sure whether you can remove any given vowel, leave it in. You'll quickly learn when you can omit vowels and when you can't. Another good rule of thumb is that any leading vowel which is not silent, and not the letter 'a', should be written down.

Once you get used to doing this, you can start using apostrophes to signify important places where there is a (usu. soft) vowel (most often the letter o in my experience, but can be any vowel). This will allow you to omit more vowels without creating uncertainty and make make it easier to read if used in some of the places where you had previously ommitted a vowel. When reading back, apostrophies should be pronounced merely as very brief gaps between the letters. "pstrphs shld b prnncd" probably would make no sense reading it later. " 'pstr'phs sh'ld b pr'n'ncd" will be much more readable, without significantly more work.

Another tip is that any long word you've already used several times can probably be shortened to the first few letters and a '~'. The reason for the ~ instead of a period is that a period is an ending mark, while a ~ is just a simple squiggle that you can easily learn to treat as a continuing mark. Also a period is percieved as signifying a preestablished abbreviation, where a ~ is on-the-fly, undefined laziness that is inferred. You know what abbr. means, but without the context, you have no clue what apo~ means (apothem in this case, but could, for example mean apothocary). I used this all the time in biology, writing things like n'c~ for nucleus, phot~ for photosynthesis... Any time there's a long word used repeatedly, you can probably reduce it to the first 2-4 letters and a ~. If there are multiple words that start the same that would confuse you, you can follow the ~ with 1-3 letters of the ending suffix to distinguish between them. (ex chl~phl vs chl~pl (chlorophyl vs chloroplast)) Keep in mind that you can only do this where you've already established what the notes are about and have used the shortened word several times. Reading through the notes it'll be about as easy, even though it might make it a bit harder to skim for something specific.