Practically, the widow's mite wasn't worth counting; it could do no useful work. To the eyes of Jesus, however, it was THE most significant. God measures what remains after the gift is given. Her gift was 100% - was it foolish investment? Impractical? A sign of dementia? But her gift mattered to the Master. The legacies of philanthropists fade, but since Christ's words will never pass away, this widow's legacy echoes forever.
Now let's preach Law: you should give 100% too! (Oops...bad theology!) Rather, try the Gospel angle: are we giving from excess (what we can spare, so it doesn't hurt us at all), or from poverty ("giving till it tickles," as Steve Green would say)? Can we spare it? Is it impractical, faith-based/ Is it based on others' expectations? Is it discreet? There's a warning here too, about the ostentatious law-teachers who sought only praise for giving.
We're all like this, for the Old Adam is a first-rate exhibitionist. Yet we have Christ as well, so our motives are for God - we seek no praise from others or from ourselves. Jesus sees faith in action, for which love is the motivation. With her pennies for heaven, the widow's mite points us to Christ, who gave 100% of Himself FOR YOU. His blood was offered to God for your redemption. Through this blood, we have an eternal inheritance! Therefore, we do all out of love.
Soli Deo gloria.