If you use Americanisms just to show you know them, people may find you a tad tiresome, so be discriminating. Many American words and expressions have passed into the language; others have vigour, particularly if used sparingly. Some are short and to the point (so prefer lay off to make redundant). But many are unnecessarily long (so use and not additionally, car not automobile, company not corporation, court not courtroom or courthouse, transport not transportation, district not neighbourhood, oblige not obligate, rocket not skyrocket, stocks not inventories unless there is the risk of confusion with stocks and shares). Spat and scam, two words beloved by some journalists, have the merit of brevity, but so do row and fraud; squabble and swindle might sometimes be used instead. The military, used as a noun, is nearly always better put as the army. Gubernatorial is an ugly word that can almost always be avoided.
Other Americanisms are euphemistic or obscure (so avoid affirmative action, rookies, end runs, stand-offs, point men, ball games and almost all other American sporting terms). Do not write meet with or outside of: outside America, nowadays, you just meet people. Do not figure out if you can work out. To deliver on a promise means to keep it. A parking lot is a car park. Use senior rather than ranking, rumpus rather than ruckus,and rumbustious rather than rambunctious.
Put adverbs where you would put them in normal speech, which is usually after the verb (not before it, which usually is where Americans put them). Choose tenses according to British usage, too. In particular, do not fight shy—as Americans often do—of the perfect tense, especially where no date or time is given. Thus Mr Bush has woken up to the danger is preferable to Mr Bush woke up to the danger, unless you can add last week or when he heard the explosion.
Prefer doctors to physicians and lawyers to attorneys. They rest from their labours at weekends, not on them and during the week their children are at school, not in it.
In an American context you may run for office (but please stand in countries with parliamentary systems) and your car may sometimes run on gasoline instead of petrol. But if you use corn in the American sense you should explain that this is maize to most people (unless it is an old chestnut). Trains run from railway stations, not train stations. The people in them, and on buses, are passengers, not riders. Cars are hired, not rented. City centres are not central cities. Cricket is a game not a sport. London is the country's capital, not the nation's. Ex-servicemen are not necessarily veterans. In Britain, though cattle and pigs may be raised, children are (or should be) brought up.
Make a deep study or even a study in depth, but not an in-depth study. On-site inspections are allowed, but not in-flight entertainment. Throw stones, not rocks, unless they are of slate, which can also mean abuse (as a verb) but does not, in Britain, mean predict, schedule or nominate. Regular is not a synonym for ordinary or normal: Mussolini brought in the regular train, All-Bran the regular man; it is quite normal to be without either. Hikes are walks, not increases. Vegetables, not teenagers, should be fresh. Only the speechless are dumb, the well-dressed smart and the insane mad. Scenarios are best kept for the theatre, postures for the gym, parameters for the parabola.
Grow a beard or a tomato but not a company. By all means call for a record profit if you wish to exhort the workers, but not if you merely predict one. And do not post it if it has been achieved. If it has not, look for someone new to head the company, not to head it up.
You may program a computer but in all other contexts the word is programme.
Try not to verb nouns or to adjective them. So do not access files, haemorrhage red ink (haemorrhage is a noun), let one event impact another, author books (still less co-author them), critique style sheets, host parties, pressure colleagues (press will do), progress reports, trial programmes or loan money. Gunned down means shot. And though it is sometimes necessary to use nouns as adjectives, there is no need to call an attempted coup a coup attempt or the Californian legislature the California legislature. Vilest of all is the habit of throwing together several nouns into one ghastly adjectival reticule: Texas millionaire real-estate developer and failed thrift entrepreneur Hiram Turnipseed...
Do not feel obliged to follow American fashion in overusing such words as constituency (try supporters), perception (try belief or view) and rhetoric (of which there is too little, not too much—try language or speeches or exaggeration if that is what you mean). And if you must use American expressions, use them correctly (a rain-check does not imply checking on the shower activity).