But it's very important to be clear what counts as alliteration and what does not.
There are several common misconceptions. So let's clear them up, and add a few other important facts.
Proper alliteration is NOT a repetition of letters, it is a repetition of sounds.
For example, fish and physics alliterate because they begin with the same consonant sound (f) - even though the initial letters are different.
Conversely, tin and thin do not alliterate, because they begin with different consonant sounds, even though they start with the same letter.
Alliteration is NOT just repeating consonant sounds at the beginning of words. What matters is the strongest, stressed syllable of a word. The only consonant which counts is the one that starts the syllable with strongest stress.
For example, below the belt is NOT a good alliteration, because stress naturally falls on the second syllable of below, so you would have to alliterate on l not on b.
On the other hand, above the beltis a good alliteration, because the stressed syllables both start with b.
Vowels alliterate with other vowels. For example, a phrase like ultimate evil alliterates because both stressed syllables start with a vowel.
Some special cases:
In the best usage, the consant s (when followed immediately by a vowel) does NOT alliterate with the consonant clusters sp, st, or sk, or with similar but distinct sounds like sh.
In some older forms of alliterative poetry, words starting with h alliterate with words starting with a vowel. This doesn't work in my dialect of English, which never drops an h. You will need to judge this point for yourself.