Keep salt content to a minimum to satisfy your sample stability/solubility requirements.
Keep in mind that a salt concentration greater than 100mM typically leads to degradation of spectral quality and spectrometer performance.
Solvent
Sample must contain a certain amount of deuterated solvent (usually D2O).
Recommended H2O:D2O ratio is 90:10 or 95:5.
Sample filtration
It is strongly recommended to filter your samples.
Protease inhibitor
If you suspect protease problems, or if you want to make sure your samples last for a while, use a "protease inhibitor cocktail".
Note that there are several small peptides in these cocktails. Since they are not deuterated, they will show up in you 1H-only spectra.
The Complete Guide for Protease Inhibition from Roche Molecular Biochemicals (PDF file)
Contaminants
NONE
Temperature for data acquisition
Typical: 2 - 50 C
Monomeric state
Sample should be unaggregated under the NMR sample conditions
