1 = t (think "one vertical stroke in letter") Also d since d is a voiced t
2 = n (two verticals)
3 = m (three verticals)
4 = r (fourth letter in "four")
5 = l because l is Roman numeral 50.
6 = j because j looks like a backwards 6. Also ch, sh, soft g, etc.
7 = k because a k can be made from two sevens. Also hard g and hard c.
8 = f because handwritten f has two loops like an eight. Also v, ph.
9 = p because p looks like a backwards 9. Also b
0 = z because z is the first letter of zero. Also s, soft c.

In my two rats John Shar's reduces to John Shar (don't ask) and kitchen police reduces to KP.