Well, that might depend on what you consider improved genetics. Better yield, better disease resistance, better flavor. A good place to start is with what some call "landrace breeding". In a nut shell you just get a bunch of seeds of a particular species and plant it all together, then save what yields, grows, tastes the best. Doing that for a period of years in many cases results in a crop that is selected for your climate, soil and tastes.
A forum member here has published a book on the topic
Lofthouse Landrace Breeding that should give you a good idea of how to get started. You could also just search landrace or landrace breeding here on the forum and find lots of good info on the topic.
When I start a new project I generally pick some traits that are important to me and try to accumulate as many varieties as possible that have one or more of those but are as different as possible in other traits. For example I began a landrace of cowpeas a few seasons ago. I prefer more bushy growth over large vines and short maturity time over full season. I just purchased or traded for about 20 varieties that had those traits and disregarded things like size or color of the peas. Now that the have mixed up some in coming years I will may select for more vigorous growth, preferred flavor and so on.
With crops that don't easily cross pollinate by wind or insects you may have to take a more hands on approach to get your initial crosses but once you have them they should produce all kinds of new types that you can select from.
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