Before I show them the endings, I like to point out, that tener and estar are going to rhyme, and that they have to pay attention to poder and poner. The best advice to students is to tell them that they need to go back and make a list of all the verbs in their glossary pages after each chapter and write them in a list as follows: yo form, present, tu' form, present; infinitive; yo form, preterite (e.g., tengo, tienes; tener; tuve). If they do this, they will be learning the verbs along with their individual quirks in the two tenses that will be the base for two more very important moods and tenses -- present subjunctive and imperfect subjunctive. If you, as their teacher, incorporate this format into quizzes, that is, test them on these forms, they will improve!
As for dar, I tell them it acts as if it were a regular -IR verb. The reason is because in Latin, it was a reduplicating verb in the perfect tense, and its conjugation began as dedi, dedisti... you can see the pattern. The first intervocalic d dropped out over time. I only offer this explanation if I either feel it will help - which it usually does not. The other reason is so they stop asking why as language learners and start asking what and how.
Ser and ir are in a class by themselves and oddly are the same in the preterite. There are reasons for that too, all having to do with the evolution of Spanish. However, if you have scared them off with an easier explanation about dar, you will probably be thanked for simply pointing it out and saying that this way, there is less to memorize. At this point, I write the conjugation of ser/ir and then tell them that these two verbs are in a class by themselves, but that the rest of the verbs that are irregular in the preterite have one set of endings in common. Then I pick a verb from the list, write its conjugation in the six-holed paradigm and ask them randomly to tell me what each form means.
The last thing to point out is that the final syllable of the first- and third-persons singular in the preterite is not stressed. A side-by-side comparison always helps.
I then proceed to do drills with verbs on the list, with the pass-off in English. If the class is ready, I tell them that I will mix in some regular verbs too.
Before I go, I tell them I will quiz all types of verbs in the preterite within three class meetings. Then I keep my promise - on the second meeting so I can catch procrastinators -- and have time to administer a merciful re-test on the third day after this lesson.