Hi, here is how I initially memorised the principal parts, in case helpful—I first memorised the principal part patterns on pages 390–2 of Mastronarde 2nd edition, and then, when I went through Mastronarde, next to each list of principal parts scattered throughout the chapters, I made notes of any deviations against the patterns or selection of variants (usually only a few words needed for each verb), enough to be able to regenerate the principal parts from (1) the relevant pattern on pages 390–2 plus (2) my notes on deviations or selection of variants.
I mentioned this on an earlier thread: http://discourse.textkit.com/t/learning-principal-parts/17155/1 , e.g. you can generate yourself the principal parts of ἄγω using (1) the pattern for velar plosive verbs on page 391 (-ττω -γω -χω -κω, -ξω, -ξα, -χα or -γα, -γμαι, -χθην or -γην), plus (2) my notes scrawled next to the ἄγω principal parts on page 51 (“3 in 2nd aor. ἤγαγον, 4 and 6 asp. versions”), where the numbers refer to the principal parts. (You also need to know the rules for augment and reduplication, but that’s the easier part I think.)
I also made mnemonics (not “historical development” explanations in many cases, but pure mnemonics) for verbs that consistently deviate from the pattern, e.g. my notes for ἐθέλω on the same page (page 51) are “2–4 as if from *ἐθελέω, 5–6 lacking”, as a way of remembering the -η- in parts 2–4: even if this is not the “true” explanation for the -η- extension, it sticks in my mind as a mnemonic, and so it’s then easy to generate parts 2–4 from the pattern for -έω verbs on page 390 (-έω -ήσω -ησα -ηκα …). I have similar notes for many other verbs (e.g. for δοκέω on page 112, my notes scrawled in my copy of Mastronarde are “2–3 and 5–6 as if from *δόκω, 4 lacking, 6 asp. version”, which allows you to regenerate its principal parts using the pattern for velar plosive verbs noted in my previous paragraph above.
I won’t bore you with many examples because I hope the above gives the general drift, but I did find engaging with principal parts in this way actively (rather than just using memorisation techniques and tools) to be quite helpful for learning the parts of verbs with less “typical” patterns, e.g. my notes for καλέω on page 152 are “1–3 as if from vowel verb rather than in -έω (with intervoc. -σ- dropped in 2), 4–6 from zero-grade -κλ- with -έω pattern endings”.
To drill principal parts, I would then to regenerate all the principal parts using my memory of the patterns on pages 390–2 plus my notes for each verb.
Later on, when I stopped the “drilling” stage of learning and began just focusing on reading, I found it more helpful to focus on the “true” explanations for deviations from patterns rather than my mnemonics above (e.g. the “Historical developments” section of the Cambridge grammar, pages 16–34, as well as the explanations in Mastronarde on pages 386–9). Nevertheless, at the earlier stage of my learning, I found the exercise of noting deviations against the patterns helpful—far more so than simply using flashcards etc. for memorising the principal parts, which wouldn’t separate out verbs with a “typical pattern” from those with deviations from the “typical pattern”.
Cheers, Chad