For both approaches, the basic seed saving techniques remain the same. The difference lies in the selection of which plants to save seed from, and the method that is chosen depends on the goals of your seed saving efforts. If it is desired to retain the 'true type' characteristics of a variety, a maintenance approach should be taken, whereas if you desire to develop or 'improve' the varety to have altered characteristics or greater suitability to your conditions, then a modification approach should be taken. In practice, there's no reason why you can't do both at the same time, but seeds of each approach should be kept separate because of their different purposes.
Let's use peas as an example.
Say you have an heirloom variety of peas that you are saving seeds from. The characteristics of this strain have been selected for generations, for various reasons. Because of this, it could be seen as important to preserve these characteristics. This could be especially relevant if you are returning your seeds to a local seed savers network. Also in the case of heirloom varieties many people like to try and preserve them the way that they were many years ago.
However, you may also wish to further adapt the variety to your own conditions, or select for characteristics that you prefer.
It should be noted here that we are talking about 'within variety' selection, as opposed to creating 'crosses' between different varieties of the same species. So in this case we are talking about growing a group of plants from the same open-pollinated seed stock, that are isolated to the necessary distance from any other varieties, depending on how they are pollinated (wind/insect pollinated plants require greater distance between them than self-pollinating plants to avoid unwanted crossing).
Regardless of which approach is used, rogueing should always be practiced. This is the process of removing any plants from your crop which are unhealthy, lacking vigour, have bolted to seed early, exhibiting signs of disease, stunted, detrimentally mutated, and so on. Seeds should always only be saved from healthy plants. This is the most fundamental form of 'selection' undertaken by a seed saving gardener.
MAINTAINING A VARIETY
If you are wanting to maintain a variety, you need to save seeds from plants that are 'true to type' / 'normal' for the variety. In other words, you'll be saving the seeds from the middle of the spectrum of variation that you see within your group of plants. Of course, healthy plants should always be selected for seed saving, but what we are talking about is the general characteristics of the plant - height, pod size, bushyness, etc.
Using this approach, you're avoiding saving things that are at the 'extreme' ends of the spectrum of observable traits, even if these ends of the spectrum seem 'better' in your own opinion.
IMPROVING / MODIFYING A VARIETY
It may be that you have a desire to modify, or 'improve' a variety to better suit your growing conditions or preferences. By saving seeds from plants that lie at a a particular end of the spectrum of traits seen in your group of plants, you can begin to further 'breed' the variety. Many changes happen slowly and it can take at least a few seasons to make any drastic changes for some varieties.
For example, you may want to have pea plants that grow taller, or have bigger peas, or more peas in each pod. Or you may want ones that can be sown earlier in the winter for an extra early spring crop of peas.
To do this, you will need to carefully select seeds from plants that exhibit the characteristics that you are looking for. I emphasise 'plants' because it is the whole plant that you need to look at - not just the individual fruits or pods. So say you are working with a pea variety that normally has around 7 peas per pod but you are aiming to increase this to 8 or 9 peas per pod. In your first year, most of the plants will have 7 peas per pod. Some plants might have 5 or 10 pods with 8-9 peas in them whereas others may have just the odd fluke pod that has 8 or 9 peas. You need to be sure to save the peas from the plants which have the most pods with a higher number of peas in them.