To successfully counter offer salary:
Research what companies at a similar stage are paying for people with your level of experience to perform similar job duties and responsibilities
Confirm that an offer is negotiable
If an offer is negotiable make a counter offer by expressing your enthusiasm for the position and your gratitude for the opportunity to negotiate
Present a specific dollar value counter offer
Present quantified reasons as to why the specific dollar value amount reflects your value in the market
If there are budget constraints be prepared with areas of compromise such as additional paid time off (PTO) or a larger signing bonus
When you reach an agreement make sure to get all details in writing