How to determine a landing position in Kerbal Space Program

You can do this with Action Groups.

This youtube tutorial provides a good example of how to do exactly what you want:

In short, once you have the solar panels added to your ship:

1. Click on the "Action Groups" icon at the top of the screen. It's the blue button that looks like two gears.

2. Select "Custom1" or whichever number you would like the action to bind to.

3. Click on the solar panels you would like to extend/retract when the key is pressed.

4. Then select "Toggle Panels" (or "Extend Panels"/"Retract Panels", if you only want this action to work in one direction.)

Now when your ship is launched, you can use whichever number key you bound this action group to to toggle your solar panels.

First of all, transmitting results doesn't make you lose anything. Transmitting and then repeating the experiment and returning always makes you end up with more science points in the end than just performing and returning the experiment once.

In the current version (23.5) each command pod can store exactly one surface sample per biome. However, the number of biomes isn't limited, so you can pick up samples from multiple biomes during one mission and store them all in one pod. To gain more science out of each "Surface sample from [Object]'s [Biome]" activity, you need multiple samples. When your lander has multiple command pods (the hitchhiker pod counts as a pod for this purpose), it can store a sample of each biome in each of them, so you can retrieve more science in one mission. However, the returns are diminishing.

Each returned sample gives you 75% of the remaining science points. A second sample gives you 75% of the remaining 25%, so two samples give you a total of 93.75% and three samples 98.4%. You can never reach 100%, no matter how many samples you return. That means a second pod on one lander might be justified, but more pods are usually a waste of payload mass.

However, when you take a sample, transmit it, take another sample, and return it, you still obtain 80% of the maximum science points which is almost as much. Considering that an antenna and a few batteries and solar panels have a lot less mass than a second pod and that you likely carry that stuff anyway to transmit crew reports (from which you can store only a single one per pod), this might be the more economical option.