Getting an even tan comes down to two things: which bikini you wear and how often you rotate. The rest — oils, timing, SPF level — matters less than most people think.
String bikinis leave the fewest marks. Triangle tops tied loosely can still leave a thin line across the shoulder blades, but shifting the neck tie slightly every hour or so makes it mostly disappear. Side-tie bottoms let you loosen the waistband when lying down, which softens the edge of the line.
Brazilian cut bottoms are particularly good for tanning. The high leg line means the tan extends further up the hip without a hard edge. Less fabric in the back means fewer lines to manage. This is part of why the style developed in Brazil in the first place — not just silhouette, but function.
Flipping matters more than most people realize. The mistake isn't forgetting to turn over — it's waiting too long. By the time you feel like you should flip, you've already been in one position long enough to set the line. Twenty minutes front, twenty minutes back, then repeat from the start.
Apply sunscreen before you put on your bikini. Let it absorb for a few minutes while you're still undressed, then dress. The edges of the fabric sit against already-protected skin, which softens the contrast where fabric meets skin.
The one line that's genuinely hard to avoid is from a halter neck tie. If you want a clean shoulder and chest result, spend at least part of your session strapless or untie the neck while lying face down. There's no trick to fix it otherwise — moving early is the only reliable answer.