Travel light. Porters are more or less extinct these days (and very expensive where you can find them). Also, if you're traveling around the country by car, train, or bus, large, heavy luggage is more of a burden than anything else. Save a little packing space for things you might buy while traveling.
In Scotland casual clothes are the norm, and very few hotels or restaurants insist on jackets and ties for men in the evening. It is, however, handy to have something semi-dressy for going out to dinner or the theater. For summer, lightweight clothing is usually adequate, except in the evening, when you'll need a jacket or sweater. A waterproof coat or parka and an umbrella are essential at any time of year. You can't go wrong with comfortable walking shoes, especially when you're climbing Edinburgh's steep urban hills or visiting Glasgow's massive museums. Drip-dry and wrinkle-resistant fabrics are a good bet because only the most prestigious hotels have speedy laundering or dry-cleaning service. Bring insect repellent if you plan to hike.
Some visitors to Scotland appear to think it necessary to adopt Scottish dress. It's not unless you've been invited to a wedding, and even then it's optional. Scots themselves do not wear tartan ties or Balmoral "bunnets" (caps), and only an enthusiastic minority prefers the kilt for everyday wear.