When a top-whorl spindle doesn't have a notch, it can be tough to control -- as the spindle fills, the yarn that comes around the whorl tends to slip along the edge. Enough that many spindlers have taken triangle files to carve a groove in their spindle. But, that isn't always desirable. Luckily, I know of two other solutions.
Velcro -- put a velcro dot on the edge of the whorl, or somewhere the yarn will go over. You want the toothy side of the velcro, so the yarn will stick to it. This can rough up the yarn a bit, but it does give you a "velcro-notch".
Starter-notch -- there's no formal name for this. But, it's a combination of techniques -- first, you get to see how to start a spindle without a leader; then, you use that "starter" as a notch, albeit a sticking-out notch. This needs pictures, so please see the diagrams in my flickr set, Spinning on a Notchless Spindle:
(click any item for details)
1. fluff out end of fiber supply
2. catch hook in fluffed out end and start twirling
3. keep yarn on hook, wrap new yarn around whorl
4. then around shaft going down shaft
5. then back up making X's
6. then come back through the hook
7. and start to spin again
8. after a few spinnings, the Xs pile up on themselves in a cop.
Click on the picture to be taken to my flickr set -- there is more information in each individual caption, I've only abbreviated things in the list above.
In particular, this style of spinning is terrific for the Victorian silk spindles: the Natalie, the Lady Ann, the Will Taylor Victorian, and the Asciano Lady; those and more of my own silk spindles shown in this picture (for details click on the picture).
To see this tutorial shown on a Natalie silk spindle, see my flickr set, Spinning on the Natalie Spindle.
For more Ask The Bellwether posts about spindles, see the Spindles category.
Comments or questions on spinning on a notchless spindle? Feel free to comment here, on flickr, or contact me.