The methods, ranked by durability
Handing the papers to the defendant personally is the most defensible method. Substituted service — delivery to a person of suitable age and discretion at the defendant's home or workplace, followed by a mailing — works but requires the follow-up mailing to be done correctly and on time. Affix-and-mail is a last resort with its own due-diligence requirement. Service by publication requires court permission and is rarely granted casually.
Serving companies
Corporations, LLCs and partnerships are served through channels defined by statute, which may include an officer or managing agent, or the Secretary of State. Serving a receptionist who is not an authorized agent is a common and fatal error. Confirm the entity's exact legal name and registered address before sending a process server; suing a trade name that does not legally exist creates its own problem.
Deadlines after filing
Service must be completed within the statutory period after commencement, and proof of service filed with the court. Missing the window can mean dismissal, and if the limitations period has expired in the meantime, the claim may be gone. If service is proving difficult, ask the court for an extension before the deadline rather than after.
Attacking service as a defendant
If you were never served, or were served in a way the statute does not allow, raise it immediately — in the answer or by a timely motion. Personal-jurisdiction objections are easily waived by delay or by litigating the merits first. Where a default judgment has already been entered on bad service, a motion to vacate is the usual route, and courts take genuine service defects seriously.