Tip number 1: Buy more/twice as many lights as you think you will need, otherwise you will get a picture in your mind based on a picture you saw that looked like this:
but your install will look like this:
See the pictures below for examples of "get more lights than you think you need" and color tone variances based on size, density, and manufacturer:
LED vs Incandescent: LEDs are more expensive, but I have found them to be less fragile, and more reliable over time. They also use significantly less energy, so you can string more of them together. If you are starting out, I would recommend LEDs.
Quantity: As I indicated earlier, buy more than you think you need. Most images that you see in marketing materials and online photos of nicely decorated houses have a relatively dense use of lights. Even measuring may not give you an accurate number, depending on what "look" you are trying to achieve. An example: We use 4x6 net lights. I have about 225 cubic feet of bushes on one side of the house. Nine of the 4x6 nets are 216 cubic feet, and you might think they would offer enough coverage. I am using 12 nets, 288 square feet, that's 63 additional cubic feet, and I think I could easily order two more nets. (You can see the bush lights in a photo below.)
Bulb Color: The obvious options here are warm white, cool white, and multi color. But even if you look at just warm white, different manufacturer's will have different interpretations of what warm white looks like - do they have a white, orange, or yellow tint.
Bulb Size: Common LED bulb sizes are 5mm and 3mm. Different bulb sizes (and even the color of the wire) will affect the color and brightness of the lights. The shape of the bulb will also affect the size of glow area around the light. This can also have an impact on the perceived "color" of the lights.
Features: It can be hard to find lights that do not have all kinds of animation features. Be careful that the lights you choose do what you want them to. For example, maybe you want steady lights, maybe you want twinkling lights. If you turn your lights on via a switch, what happens when the power is restored, do they go back to the settings you wanted, or do they default to something else?
Extendable: Not all lights can be extended. Be sure to consider this.
Density: Not all light strands are created equal. Some strands will space out a bulb every 6" or 4" or even 1". Some will have more than one light per inch (often called "cluster" lights). Combined with bulb size, this can change the look as well.
Brand: As opposed to recommending a specific brand, I am going to suggest that you find a good brand (try www.wirecutter.com) and stick with it. Keeping the same brand will help give you a consistent look across your lights: color, brightness, "glow" size/shape, rate of twinkle, etc.
Two meteor shower light strands (each strand has 3 drops) from the garage roof ...
2 strands, 6 drops, 256 lights
... and here are four meteor shower light strands ...
4 strands, 12 drops, 512 lights
... and here are twelve meteor shower light strands.
12 strands, 36 drops, 1,536 lights
These two photos show examples of light quantity, density, and color tone.
In the foreground of the image at the right, you see a trunk wrapped with two 2x6 trunk-wrap net lights strands (200 lights). It "works", but compare that to the trunk wraps in the image above, using string lights (3,000 lights per trunk).
Note the color tone of the lights on the bushes and columns compared to the trunk wrap and picture window. Those are all "warm white" lights, but from different manufactures, with different densities, and different color wire. They don't match, and the color tone difference is even more obvious in person.
Invest in Zip Cord — zip cord is inexpensive electrical cord (similar to lamp cord) that can be customized to any length and allows you to place any number of outlets (called vampire plugs) anywhere along the line that you build, without any "wiring".
For example, you can make a custom length cable with an outlet 20ft from the end, then another outlet 25ft from the end, then four outlets in a row, each six inches apart, 78ft from the end. This can save you from buying several (more) expensive extension cords that are too long and various splitters. The components install by snapping together without any tools.
I am building a custom extension cable that will snake around the entire perimeter of my house and yard, placing inline outlets exactly where I need them to connect various trim and tree light strands along the way.
If you don't invest in Zip Cord, buy lots of extension cables and splitters, both heavy duty outdoor extensions, and lighter duty indoor extension cables. When you don't make your own cables using Zip Cord, be prepared to hide the excess cable somewhere.
Buy a bunch of pigtail splitters, both to connect multiple items as well as to branch off in different directions.
Floral Wire comes in handy for tying garland and lights to just about anything
Binder clips are handy for quickly and temporarily clipping to objects and then binding lights/garland to them.
Cup/Ceiling Hooks with safety/wind clips. These clips allow for quickly stringing lights, but they won't come undone when you don't want them to.
Extension cords with outlets spaced every 5 FT - these are handy year round!
Don't forget storage! You will need storage for lights, trees, extension cords, garland, decorations, etc. I have storage boxes with labels that indicate what is in them. Each box is numbered based on the order that I install them.
I recommend having two or more ladders, or raised work platforms. If you have a stretch of lights to hang along your roof or windows, it is handy if you have more than one ladder, especially if you are installing more than one run along a path, or need to straighten or adjust things as you go. Otherwise it is a constant up-and-down the ladder, reposition ladder, repeat, all the while trying to keep your lights untangled.