High voltage sampling with microcontroller potential divider affecting signal
You need to buffer the signal before dividing it down. Op-amps are the usual way of doing that. Depending on the precision you need, an instrumentation op-amp costs a little more but has a lower offset voltage and tends to be the most linear over range.

The usual configuration is to connect the signal to the positive input of an op-amp, perhaps with a high value resistor for protection. The input of the op-amp is very high impedance. The output is then divided with resistors, and optionally fed into another op-amp buffer to avoid problems stemming from the impedance of the ADC.

The following schematic demonstrates a divide by 2: