A bright hello to everybody on the list, :-)

I am asking you the following to get an idea how other people handle
this issue with their conlangs. Maybe it will help me with my own
one. :-))

Once decisions have been made about the phonology and about the basic
layout of grammar, how do you assign the neccessary affixes and/or
particles? Since these tiny bits of language will be very prominent in
all kinds of sentences, they should be chosen carefully, shouldn't they?
I think that they essentially constitute the sound of a language, especially
when it comes to make rhymes in poetry.

Furthermore, if a particular affix has (for example) been assigned to
an inflectional ending, then it may be of limited use for a regular word
ending. Umlaut or ablaut can take over the function of a suffix that can
not be appended - German is quite full of pertinent examples - but the
decision on grammatical morphology obviously has direct implications
for regular word-building itself.

So, how do you select these tiny but important parts of your languages?
Are there any combinations which especially "mix" well with adjacent
words? For example, I thought about the Arabic article "al" and found
out that it goes unobtrusively with most of the words that I tried. Are
there some universals for basic grammatical affixes such as prefixes,
suffixes and short particles?

I am looking forward to your answers. Thank you for taking the time! :-)))