1. In the absence of a miswaak, the finger may be used as a substitute.
عن أنس رضي الله عنه قال قال رسول الله صلى الله عليه وسلم يجزئ من السواك الأصابع (التلخيص الحبير 1/104)
Hazrat Anas (radhiyallahu anhu) reports that Hazrat Rasulullah (sallallahu alaihi wasallam) said, “The fingers can suffice as a miswaak (in the absence of a miswaak).”
2. The miswaak should not exceed a hand-span in length, and should be equal to the small finger in thickness.
3. Any stick that is useful for cleansing the mouth and is not harmful or poisonous can be used as a miswaak. The best miswaak is from the peelu tree (salvadora persica) and then the olive tree.
عن معاذ بن جبل رضي الله عنه قال سمعت رسول الله صلى الله عليه وسلم يقول نعم السواك الزيتون من شجرة مباركة تطيب الفم وتذهب بالحفر وهو سواكي وسواك الأنبياء قبلي (مجمع الزوائد، الرقم: 2576)
Hazrat Mu’aaz bin Jabal (radhiyallahu anhu) reports that Hazrat Rasulullah (sallallahu alaihi wasallam) said, “How excellent is the miswaak taken from the olive tree! It is from a blessed tree, cleanses the mouth and removes gum sicknesses and maladies (and removes the yellowness of the teeth). It is my miswaak as well as the miswaak of the previous Ambiyaa (alayhimus salaam).”
عن ابن مسعود رضي الله عنه كنت أجتني لرسول الله صلى الله عليه وسلم سواكا من أراك (إعلاء السنن 1/75 ، التلخيص الحبير 1/95)
Hazrat Abdullah bin Mas’ood (radhiyallahu anhu) reports, “I used to break twigs from the peelu tree (salvadora persica) for the miswaak of Rasulullah (sallallahu alaihi wasallam).”