As a general rule, peptides should first be dissolved in distilled, preferably sterile water. If solubility still remains a problem, try the following steps:

Sonication is helpful to dissolve the peptide
Small amounts of dilute(10%) aqueous acetic acid for basic peptides or aqueous ammonia for acidic peptides may help dissolution of these peptides.

It is also recommended that the peptide be dissolved to the highest possible concentration, and then diluted with water or buffer to the working concentration.(Note: The buffer should be added only after the peptide is completely in solution form because salts may cause aggregation).

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