Start out with a juicy ink pad. If the pad you're using hasn't been reinked in a while, this can often be the gremlin.
Sometimes, stamping with a chalk ink or a pigment ink will improve the look substantially.
We used to advise each other to use a pencil eraser to remove 'new stamp' residue. I haven't done that in years ... leaning more toward finding the right brand of ink in my stash that works well with the stamp. Here in my stampin' zone, it seems that some clear stamps respond differently to various brands of ink.
Hang in there ... you'll get to the bottom of this with perseverance.